Water-Absorbent Resin Composition, Method of Manufacturing the Same, and Absorbent Article

ABSTRACT

In one embodiment of the present invention, a water-absorbent resin composition is provided which shows only small reduction in liquid permeability and limited coloring over time or in relation to another factor. The water-absorbent resin composition (particulate absorbent agent) of the present invention contains: a polycarboxylate-based water-absorbent resin as a primary component, the resin having a crosslinked structure formed by polymerization of an acid group-containing unsaturated monomer; and multivalent metal cations (preferably, on surfaces of particles (i.e., the water-absorbent resin composition)) wherein: the multivalent metal cations other than Fe cations account for 0.001 to 1 mass % of the water-absorbent resin; and the ratio of the Fe cations to the multivalent metal cations other than the Fe cations is less than or equal to 5.00 mass %.

TECHNICAL FIELD

The present invention relates to water-absorbent resin compositions,manufacturing methods therefor, and absorbent articles containing awater-absorbent resin composition.

BACKGROUND ART

A water-absorbent resin or a water-absorbent resin composition has beenconventionally widely used as a primary component of paper diapers,sanitary napkins, incontinence pads, and other hygienic materials(absorbent articles) for the purpose of absorbing a body fluid (urine,blood, etc.).

The water-absorbent resin composition contains a water-absorbent resinas a primary component. Known examples of the water-absorbent resincomposition include: crosslinked products of partially neutralizedproducts of polyacrylic acid, hydrolysates of starch-acrylonitrile graftpolymers, neutralized products of starch-acrylic acid graft polymers,saponification products of vinyl acetate-acrylic ester copolymers,crosslinked products of carboxy methyl cellulose, hydrolysates ofacrylonitrile copolymers or acrylamide copolymers and crosslinkedproducts of the hydrolysates, crosslinked products of cationic monomers,crosslinked isobutylene-maleic acid copolymers, and crosslinked productsof 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid.

Recent paper diapers and like hygienic materials have increasingly highperformance and a decreasing thickness. Thickness reduction is attemptedwhile increasing absorption and securing leak prevention by increasingthe amount or ratio (mass-equivalent ratio in absorbent article) of thewater-absorbent resin composition used:

Hygienic materials containing an increased amount of water-absorbentresin composition in this, manner are preferable in view of simplystoring more liquid. However, when actually used in a diaper, thewater-absorbent resin composition absorbs water, swells, and changes tosoft gel in the diaper. This phenomena, or “gel blocking,” reducesabsorption and causes leakage. This is undesirable.

Accordingly, the liquid permeability of the water-absorbent resincomposition has recently attracted a great deal of attention. Somedocuments, including patents documents 1 to 7 listed below, report amethod of raising liquid permeability by crosslinking the surface of thewater-absorbent resin with cations of aluminum or a like multivalentmetal. Multivalent metal cations are widely used also to modify thewater-absorbent resin other than the improvement of liquid permeability:for example, improvement of powder fluidity upon moisture absorption andimprovement, of mixing property of an aqueous liquid with thewater-absorbent resin.

It is known that aluminum sulfate generally contains Fe in an amount of0.16 to 11.5 mass % to the A1 (see, non-patent document 1, for example).

Patent document 1: U.S. Pat. No. 6,620,889, Specification (issued Sep.16, 2003).Patent document 2: U.S. Pat. No. 6,605,673, Specification (issued Aug.12, 2003)Patent document 3: U.S. Pat. No. 6,863,978, Specification (issued Mar.8, 2005).Patent document 4: U.S. Pat. No. 6,323,252, Specification (issued Nov.27, 2001)Patent document 5: U.S. Pat. Application 2005/00671 (published Mar. 31,2005)Patent document 6: U.S. Pat. No. 4,771,105, Specification (issued Sep.13, 1988)Patent document 7: U.S. Pat. No. 4,043,952, Specification (issued Aug.23, 1977)Non-patent document 1: EUROPEAN STANDARD, EN 878:2004, page 10 (Table5), published June 2004.

DISCLOSURE OF INVENTION

The inventors of the present invention, however, have found, that theconventional water-absorbent resin composition described above in whichthe surface of the water-absorbent resin is crosslinked by cations of amultivalent metal, when used in diapers and other absorbent articles,does not show desirable liquid permeability, and worse still causesunwanted coloring.

It has been known that multivalent metal cations can be used to modify awater-absorbent resin composition. In contrast, it is not known, thatthe water-absorbent, resin composition containing multivalent metalcations suffers from decreasing liquid permeability over time(especially when used to absorb urine) and coloring (especiallyprogressive coloring over time).

The water-absorbent resin composition which has liquid permeabilityimproved by the use of multivalent metal cations is usually used in highconcentration (high weight) in diapers and like absorbent articles. Thecoloring of the water-absorbent resin composition used in large amountsleads to coloring of the absorbent article and lowers the commercialvalue of the absorbent article.

If the water-absorbent resin composition colors after converting (forexample, after the manufacture of the diaper), the coloring occursduring the distribution or after the sale of the absorbent article. Thecoloring of the absorbent article may not be found until a consumer isabout to use it. The consumer would make a complaint to the manufacture,which possibly undesirably damages the consumer's trust in thecommercial good and the good's reputation in the market.

In view of the problems mentioned above, it is an objective of thepresent invention to provide a water-absorbent resin compositionsuitable for use in diapers and like absorbent articles, as well as forother practical purposes. More specifically, the objective is to providea water-absorbent resin composition suitable for use in diapers and likeabsorbent articles as well as for other practical purposes, which showslittle decrease in liquid permeability over time or in relation toanother factor and limited coloring (especially, over time).

The inventors have diligently worked to solve the problems and foundduring the course that the water-absorbent resin composition fails toshow sufficient liquid permeability when the composition is used indiapers or other urine-absorbent articles. The inventors have found thatsince the water-absorbent resin composition shows improved liquidpermeability for physiological saline and other substances which areused to evaluate the liquid permeability, the cause of the problems isthe water-absorbent resin composition changing over time or in relationto another factor in actual use of the absorbent article in such anenvironment that the composition contacts urine and that these,time-related changes lower the liquid permeability of thewater-absorbent resin composition.

Specifically, the water-absorbent resin composition containing awater-absorbent resin of which the surface is crosslinked by multivalentmetal cations shows, improved liquid permeability when manufactured. Thecomposition, however, shows poor liquid permeability after an extendedperiod of use (for example, 16 hours), especially in an environmentwhere the composition is in contact with, urine because the polymerchain of the water-absorbent resin breaks or otherwise changes over timeor in relation to another factor. Thinking of the time a diaper is worn(for example, throughout the night), the decrease in liquid permeabilityduring that period is very undesirable. For the above reasons, thewater-absorbent resin composition containing a water-absorbent resin ofwhich the surface is crosslinked by multivalent metal cations, in somecases, shows poorer liquid permeability after some time than thewater-absorbent resin composition containing a water-absorbent resin ofwhich the surface is not crosslinked.

The inventors have found another problem: the water-absorbent resincomposition colors (turns to yellow) even if the surface of thewater-absorbent, resin is crosslinked by colorless, transparentmultivalent metal cations. The coloring is barely appreciable at thetime of manufacture of the water-absorbent resin composition. Thecoloring of the water-absorbent resin composition progresses, however,with time during storage or after converting (for example, after thecomposition is fabricated into a diaper) after the composition ismanufactured.

In an effort to solve the phenomena (the decrease in liquid permeabilityand coloring of the water-absorbent resin composition), the inventorshave, found that the phenomena, become more distinct with an increasingamount of multivalent metal cations used. The inventors have furtherfound that a specific impurity (Fe cations) in the multivalent metalcations and impurities, such as methoxyphenols and furfural, in acrylicacid induce the phenomena and that the problems are solved bycontrolling the specific impurity (Fe cations) in the multivalent metalcations within a critical range (ordinary multivalent metal cationsavailable on the market contain 200 to 50,000 ppm Fe cations to themultivalent metal cations), controlling the impurities, such asmethoxyphenols and furfural, in acrylic acid within a particular range,or adding a metal chelating agent to the water-absorbent resincomposition, which has led to the completion of the invention.Especially, the inventors have identified the impurities that affect thephenomena regarding water-absorbent resin compositions obtained byaqueous polymerization, and found the content ratio for the impuritiesat which the impurities greatly suppress the phenomena.

To achieve the objective, a water-absorbent resin composition inaccordance with the present invention is characterized in that thecomposition contains: a polycarboxylate-based water-absorbent resin as aprimary component, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the multivalent metal cations otherthan Fe cations account for 0.001 to 1 mass % of the water-absorbentresin; and the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is less than or equal to 5.00 mass %.

Another water-absorbent resin composition in accordance with the presentinvention is characterized in that the composition comprises: apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; and multivalent metal cations,wherein: the multivalent metal cations other than Fe cations account for0.001 to 1 mass % of the water-absorbent resin; and the ratio of the Fecations to the water-absorbent resin is not more than 1 ppm (preferablynot more than 0.1 ppm).

The water-absorbent resin composition not only shows high liquidpermeability for physiological saline, etc., but also shows high liquidpermeability in actual use when used in, for example, absorbent articlesand shows limited reduction in the liquid permeability over time. Thatpermeability has attracted no attention at all at the time ofmanufacture of the water-absorbent resin composition. In addition thewater-absorbent resin composition shows limited Fe cation-causedcoloring, especially over time. The composition colors less duringdistribution and after sale. That coloring over time, has attracted noattention at the time of manufacture of the water-absorbent resincomposition. Thus, the invention provides a water-absorbent resincomposition which shows only small reduction in liquid permeability overtime or in relation to another factor and limited coloring (especially,over time) and which is suitable for use in diapers and like absorbentarticles, as well as for other practical purposes.

To achieve the objective, another water-absorbent resin composition inaccordance with the present invention is characterized in that thecomposition contains: a polycarboxylate-based water-absorbent resin as aprimary component, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer;multivalent metal cations; and a chelating agent, wherein: themultivalent metal cations other than Fe cations account for 0.001 to 1mass % of the water-absorbent resin; and the ratio of the Fe cations tothe multivalent, metal cations other than the Fe cations is less than orequal to 50 mass %.

Another water-absorbent resin composition in accordance with the presentinvention is characterized in that the composition comprises: apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; multivalent metal cations; and achelating agent, wherein: the multivalent metal cations other than Fecations account for 0.001 to 1 mass % of the water-absorbent resin; andthe ratio of the Fe cations to the water-absorbent resin is not morethan 1 ppm (preferably not more than 0.1 ppm).

The water-absorbent resin composition contains a chelating agent and asmall amount of Fe cations. The water-absorbent resin composition notonly shows high liquid permeability for physiological saline, etc., butalso shows high liquid permeability in actual use when used in, forexample, absorbent articles and shows limited reduction in the liquidpermeability over time. That reduction has attracted no attention at allat the time of manufacture of the water-absorbent resin composition. Inaddition, the water-absorbent resin composition shows limited Fecation-caused coloring, especially, over time. The water-absorbent resincomposition and absorbent article color less during distribution andafter sale. That, coloring over time has attracted no attention at thetime of manufacture of the water-absorbent resin composition. Thus, theinvention provides a water-absorbent resin composition which shows onlysmall reduction in liquid permeability over time or in relation toanother factor and limited coloring (especially, over time) and which issuitable for use in diapers and like absorbent articles, as well as forother practical purposes.

To achieve the objective, a method of manufacturing a water-absorbentresin composition in accordance with the present invention ischaracterized in that the method involves the steps of: (a) polymerizingan acid group-containing unsaturated monomer into apolycarboxylate-based water-absorbent resin with a crosslinkedstructure, the monomer primarily including an acrylic acid and/or a saltthereof as primary components; and (b) adding multivalent metal cationsto the water-absorbent resin in 0.001 to 5 mass % to the water-absorbentresin, the ratio of Fe cations to the multivalent metal cations otherthan the Fe cations being less than or equal to 0.50 mass %.

The method enables the manufacture of a water-absorbent resincomposition which riot only shows high liquid permeability forphysiological saline, etc., but also shows high liquid permeability inactual use when used in, for example, absorbent articles and showslimited reduction in the liquid permeability over time. That reductionhas attracted no attention at all at the time of manufacture of thewater-absorbent resin, composition. Therefore, the method enables themanufacture of a water-absorbent resin composition which shows onlysmall reduction in liquid permeability over time or in relation toanother factor and which is suitable for use in diapers and likeabsorbent articles, as well as for other practical purposes.

To achieve the objective, an absorbent article in accordance with thepresent invention is characterized in that the article is at least oneabsorbent article selected from the group consisting of a paper diaper,a sanitary napkin, and an incontinence pad and contains awater-absorbent resin composition in accordance with the presentinvention.

The article contains a water-absorbent resin composition in accordancewith the present invention. Thus, the invention provides an absorbentarticle which shows only small reduction in liquid permeability overtime or in relation, to another factor and limited coloring (especially,over time).

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating thearrangement of a device which measures the degraded liquid permeationrate of a water-absorbent resin composition.

FIG. 2 is a cross-sectional view schematically illustrating thearrangement of a device which measures the saline flow conductivity(SFC) of a water-absorbent resin composition.

BEST MODE FOR CARRYING OUT INVENTION

The following will describe embodiments of the present invention indetail. The description is by no means binding on the scope of thepresent invention. The invention can be modified in suitable manners andcarried out in other ways than the examples below within the spirit ofthe invention.

Throughout the following description, “weight” is a synonym of “mass,”“wt %” of “mass %,” and “primary component” of “accounting for more thanor equal to 50 mass % of the total.” The expression for a numericalrange, “A to B,” refers to the range not less than A and not more thanB.

Decimal 0s in mass % (wt %) are omitted unless otherwise stated. Eachmass % value however contains significant digits down to the ppm units:for example, “1 mass %” is a synonym for “10,000 ppm” (0s in “1.0000mass % “are omitted). Also, the unit, “ppm,” indicates a mass-equivalentvalue unless otherwise stated. For example, 10000 ppm means 1 mass %.

A first water-absorbent resin composition in accordance with the presentembodiment is a water-absorbent resin composition containing apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; and multivalent metal cations,wherein: the multivalent metal cations other than Fe cations account for0.001 to 1 mass % of the water-absorbent resin; and the ratio of the Fecations to the multivalent metal cations other than the Fe cations isless than or equal to 5 mass % (50,000 ppm).

A second water-absorbent resin composition in accordance with thepresent embodiment is a water-absorbent resin composition containing apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; multivalent metal cations; and achelating agent, wherein: the multivalent metal cations other than Fecations account for 0.001 to 1 mass % of the water-absorbent resin; andthe ratio of the Fe cations to the multivalent metal cations other thanthe Fe cations is less than or equal to 50 mass % (500,000 ppm).

(1) Water-Absorbent Resin

The water-absorbent resin is a water-swellable, water-insolublecrosslinked polymer which, forms hydrogel upon swelling. Awater-swellable crosslinked polymer is, for example, a crosslinkedpolymer which absorbs at least 5 times, preferably, 50 to 1,000 times,as much water as its weight in ion exchange water. A water-insolublecrosslinked polymer is, for example, a crosslinked polymer containinguncrosslinked water soluble components (water-soluble polymer) in anamount of, preferably 0 to 50 mass %, more preferably less than or equalto 25 mass %, even more preferably less than or equal to 20 mass %,still more preferably less than or equal to 15 mass %, yet morepreferably less than or equal to 10 mass % to the water-absorbent resin.Measurement methods for these values will be given in examples of theinvention detailed later.

The water-absorbent resin is preferably a water-absorbent resin having acrosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer (hereinafter, “monomer”), morepreferably a polycarboxylate-based water-absorbent resin having acrosslinked structure, in view of liquid permeability and liquidabsorption properties. A polycarboxylate-based water-absorbent resin isa water-absorbent resin having a carboxy group in its main chain as arepeat unit.

Specific examples of the water-absorbent resin include polymers ofpartially neutralized products of polyacrylic acid, hydrolysates ofstarch-acrylonitrile graft polymers, starch-acrylic acid graft polymers,saponification products of vinyl acetate-acrylic ester copolymers,hydrolysates of acrylonitrile copolymers and acrylamide copolymers,crosslinked products of these substances, denatured products of carboxylgroup-containing crosslinked polyvinyl alcohol, crosslinkedisobutylene-maleic anhydride copolymers, and mixtures of any of thesesubstances. The water-absorbent resin is preferably a polymer of apartially neutralized product of polyacrylic acid obtained bypolymerization and crosslinking of a monomer containing an acrylic acidand/or a salt (neutralized product) thereof as primary components.

The acid group-containing unsaturated monomer (“monomer”) may beacrylonitrile or another monomer which produces acid groups inhydrolysis after polymerization. The monomer is preferably an acidgroup-containing unsaturated monomer containing acid groups at the timeof polymerization. Preferred examples of the acid group-containingunsaturated monomer containing acid groups at the time of polymerizationare unsaturated monomers containing an acrylic acid and/or a saltthereof as a primary component. In the present embodiment, anunpolymerizable organic compound is preferably contained in, or addedto, the acrylic acid used to prepare the monomer in advance.Specifically, it is preferable to dissolve an unpolymerizable organiccompound in an unneutralized acrylic acid before preparing an aqueoussolution of a monomer using the unneutralized acrylic acid.

The unpolymerizable organic compound is an organic compound having nopolymerizable unsaturated bond formed with a vinyl group, an allylgroup, or the like. The present embodiment may use a 1 to 1000 ppm byweight, wherein the unpolymerizable organic compound has a solubilityparameter of (1.0 to 2.5)×10⁴ (Jm⁻³)^(1/2).

The solubility parameter (δ) is herein a cohesive energy density and canbe calculated from the following equation:

δ((Jm⁻³)^(1/2))=ρΣG/M  (Equation 1)

where ρ is density (g/cm³), G is the Holly cohesive energy constant, ΣGis a sum of cohesive energy constants of component atom groups, ρ and Gare values at a temperature of 25±1° C., and M is molecular weight.

Herein, if the solubility parameter 8 is calculated in the unit((calm⁻³)^(1/2)), the solubility parameter is appropriately expressed inthe unit (Jm⁻³)^(1/2).

For example, the value δ of solubility parameter specified in thepublications such as The Polymer Handbook, 3rd Edition (pages 527 to539; published by Wiley Interscience Publication) and Chemical handbookBasic Edition (published by the Chemical Society of Japan) is adopted.Also, as the solubility parameter of a solvent, which is not specifiedon the publications, the value δ which is obtained by substituting theHolly cohesive energy constant into the Small equation specified on page524 of The Polymer Handbook is adopted.

The present embodiment uses the monomer including the above particularcompound in certain amounts, thereby producing, with a highproductivity, a water-absorbent resin having (i) an improvedrelationship between absorption capacity and water-soluble polymer,which are conflicting properties of the water-absorbent resin, (ii)being easily controlled for polymerization reaction, (iii) being lesscolored, and (iv) being of high absorption properties. A monomer havingan unpolymerizable organic compound content of less than 1 ppm byweight, wherein the unpolymerizable organic compound has a solubilityparameter of (1.0 to 2.5)×10⁴ (Jm⁻³)^(1/2), is not preferred because ithas the difficulty in being controlled for polymerization, which iscaused by an excessive rise in temperature of a polymerized substancedue to heat liberated by the polymerization, and causes degradation inabsorption properties. Meanwhile, a monomer having an unpolymerizableorganic compound content of more than 1000 ppm by weight, wherein theunpolymerizable organic compound has a solubility parameter of (1.0 to2.5)×10⁴ (Jm⁻³)^(1/2), includes too much amount of the unpolymerizableorganic compound to achieve the object of the present invention, andmight cause the problem, e.g. odor from a resultant water-absorbentresin.

Further, the particular compound (unpolymerizable organic compound) isfinally removed by a particular heating step (e.g. drying and surfacetreatment) so that the resultant water-absorbent resin is free fromodors and other problems.

Such an unpolymerizable organic compound is used preferably in an amountof 1 to 1000 ppm by weight, more preferably 1 to 500 ppm by weight, evenmore preferably 1 to 300 ppm by weight, still more preferably 5 to 300ppm by weight, yet more preferably, 10 to 300 ppm by weight, mostpreferably 10 to 100 ppm by weight, relative to the monomer (or themonomer composition which will be detailed later).

The solubility parameter of the unpolymerizable organic compound isessentially (1.0 to 2.5)×10⁴ (Jm⁻³)^(1/2), preferably (1.0 to 2.2)×10⁻⁴(Jm⁻³)^(1/2), more preferably (1.1 to 2.0)×10⁴ (Jm⁻³)^(1/2), still morepreferably 1.3 to 2.0)×10⁴ (Jm⁻³)^(1/2), and most preferably (1.5 to1.9)×10⁴ (Jm⁻³)^(1/2).

The organic compound having a solubility parameter of (1.0 to 2.5)×10⁴(Jm⁻³)^(1/2), which is an organic compound having an excellentcompatibility with acrylic acid and having no polymerizable unsaturatedbonds, refers to a lipophilic organic compound. Of such unpolymerizableorganic compounds, an organic compound having no halogen content ispreferable, and hydrocarbon consisting of only carbon and hydrogen ismore preferable, in terms of environmental loads. Further, a boilingpoint of the unpolymerizable organic compound is preferably 95 to 300°C., more preferably 130 to 260° C. The organic compound having asolubility parameter of more than 2.5×10⁴ (Jm⁻³)^(1/2) is not preferablein terms of polymerization control and polymerization reaction.

More specifically, the unpolymerizable organic compound is at least onecompound selected from the group consisting of: heptane (boiling point:95° C.), dimethyl cyclohexane (boiling point: 132° C.), ethylcyclohexane, toluene (boiling point: 110° C.), ethyl benzene (boilingpoint: 136° C.), xylene (boiling point: 138 to 144° C.), diethyl ketone(boiling point: 101° C.), diisopropyl ketone (boiling point: 124 to 125°C.), methyl propyl ketone (boiling point: 102° C.), methyl isobutylketone, methyl-t-butyl ketone, n-propyl acetate (boiling point: 101°C.), n-butyl acetate (boiling point: 124 to 125° C.), diphenyl ether(boiling point: 259° C.), and diphenyl (boiling point: 255° C.).

Of these unpolymerizable organic compounds, preferably is at least onecompound selected from the group consisting of heptane, ethyl benzene,xylene, methyl isobutyl ketone, methyl-t-butyl ketone, diphenyl ether,and diphenyl, more preferably hydrophobic compounds, still morepreferably, aromatic compounds, particularly preferably toluene,diphenyl ether, and diphenyl, most preferably toluene in terms ofpolymerization properties and productivity and further in terms of theeffect of inhibiting oxidation and deterioration of a polymer chainafter the completion of the polymerization step.

The unpolymerizable organic compound is more preferably included in themonomer (or the monomer composition which will be detailed later) beforepolymerization. The monomer including unpolymerizable organic compoundmay be prepared in such a manner that the unpolymerizable organiccompound is added to a monomer (or the monomer composition which will bedetailed later) after the preparation of the monomer, theunpolymerizable organic compound is added to a monomer (or the monomercomposition which will be detailed later) during the preparation of themonomer, or the unpolymerizable organic compound is included in advanceor added to raw materials for a monomer (or the monomer compositionwhich will be detailed later) including acrylic acid, cross-linkingagents, water, and alkali compounds. In such preparation methods, theunpolymerizable organic compound is hydrophobic and generallywater-insoluble, and therefore is preferably dissolved or included inacrylic acid, in advance. In the present embodiment, it is preferablethat the unpolymerizable organic compound is included or added, inadvance, to acrylic acid as used in preparing the monomer. That is, itis preferable that the unpolymerizable organic, compound is dissolved inadvance in an unneutralized acrylic acid so that the unneutralizedacrylic acid is used for the preparation of an aqueous solution of themonomer.

The acrylic acid of the present embodiment preferably, contains a 10 to180 ppm methoxyphenol, more preferably 10′ to 150 ppm, even morepreferably 10 to 90 ppm, still more preferably 10 to 80 ppm, and themost preferably 10 to 70 ppm.

Methoxyphenols are compounds which have a methoxyphenol unit. Specificexamples include o-, m-, and p-methoxyphenol with or without at leastone substituent, such as a methyl group, a t-butyl group, or a hydroxylgroup. In the present embodiment, p-methoxyphenol is especiallypreferable.

If the methoxyphenol (especially, p-methoxyphenol) accounts for morethan 180 ppm, the resultant water-absorbent resin is undesirably colored(becomes yellowish or turns into yellow). If the methoxyphenol accountsfor less than 10 ppm (especially, if less than 5 ppm) as a result ofremoving the methoxyphenol (especially, p-methoxyphenol) from theacrylic acid by, for example, distilling or other refining, there is arisk of the acrylic acid starting polymerization before deliberatelydoing so. Besides, surprisingly, such low methoxyphenol content, couldslow down the rate of polymerization of the acrylic acid.

The acrylic acid used in the present embodiment may be p-methoxyphenolor another non-methoxyphenol polymerization inhibiting agent in itsmanufacturing step. Examples of effective non-methoxyphenolpolymerization inhibiting agents include phenothiazine, hydroquinone,copper salt, and methylene blue. These non-methoxyphenol polymerizationinhibiting agents, unlike methoxyphenols, disrupt the polymerization ofacrylic acid. It is therefore better if the ultimate acrylic acidcontains less of the agents. The agents account for preferably 0.1 ppmor less, more preferably 0 ppm (less than or equal to detectable limit)of the acrylic acid.

The acrylic acid used in the present embodiment preferably contains 20ppm or less furfural. With growing furfural content in the acrylic acid,polymerization time (time taken to reach polymerization peaktemperature) becomes longer, hence more monomer remains unreacted, andthe resultant water-absorbent resin contains much more soluble(water-soluble) content. To improve the physical properties andcharacteristics of the water-absorbent resin, the acrylic acid containsmore preferably 10 ppm or less furfural, even more preferably 0.01 to 5ppm, still more, preferably 0.05 to 2 ppm, and the most preferably 0.1to 1 ppm.

When the monomer contains an acrylic acid and/or a salt thereof asprimary components, another monomer may be used together. In a case likethis, the methoxyphenol content and furfural content described above aredetermined by the content in a monomer composition containing the othermonomer, an acrylic acid, and/or a salt thereof. That is, the monomercomposition contains preferably 10 to 18.0 ppm methoxyphenol and 20 ppmor less furfural,

Examples of the other monomer include water-soluble or hydrophobicunsaturated monomers, such as methacrylic acid, maleic (anhydride) acid,fumaric acid, crotonic acid, itaconic acid, vinyl sulfonic acid,2-(meth)acrylamide-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid and an alkali metal salt thereof, theammonium salt, N-vinyl-2-pyrrolidone, N-vinyl acetoamide,(meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, isobutylene, and lauryl (meth)acrylate.

When the monomer contains non-acrylic acid (salt), the non-acrylic acid(salt) monomer is contained in ah amount of preferably 0 to 30 mol %,more preferably 0 to 10 mol %, to the sum of the acrylic acid and/or thesalt thereof as primary components. The use of a non-acrylic acid (salt)monomer in that amount imparts further improved absorption properties tothe final water-absorbent resin (and water-absorbent resin composition)and further reduces the manufacturing cost for the water-absorbent resin(and water-absorbent resin composition).

The water-absorbent resin must have a crosslinked structure. Thecrosslinked structure of the water-absorbent resin may be formed eitherby the use of an internal crosslinking agent or automatically, withoutusing an internal crosslinking agent. The crosslink is formed preferablyby copolymerization or reaction of an internal crosslinking agentcontaining in each molecule at least two polymerizing unsaturated groupsor reactive groups (an internal crosslinking agent for thewater-absorbent resin).

Specific examples of the internal crosslinking agent includesN,N′-methylene bis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, glycerine tri(meth)acrylate,glycerine acrylate methacrylate, ethylene oxide-denaturedtrimethylolpropane tri(meth)acrylate, pentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol,polyethylene glycol, propylene glycol, glycerine, pentaerythritol,ethylenediamine, ethylene carbonate, propylene carbonate,polyethyleneimine, and glycidyl(meth)acrylate.

Any of the internal crosslinking agents may be used alone, oralternatively two or more of them may be used together in any suitablecombination. The internal crosslinking agents may be added to thereaction system in single batch or in multiple batches. Considering theabsorption and other properties of the final water-absorbent resin andwater-absorbent resin composition, it is preferable to unexceptionallyuse a compound containing multiple polymerizing unsaturated groups ineach molecule for polymerization if at least one or two internalcrosslinking agents are used.

The internal crosslinking agent is used in preferably 0.001 to 2 mol %,more preferably 0.005 to 1 mol %, even, more preferably 0.005 to 0.5 mol%, still more preferably 0.01 to 0.5 mol %, yet more preferably 0.01 to0.2 mol %, again more preferably 0.03 to 0.2 mol %, and most preferably0.03 to 0.15 mol %, to the monomer (minus the internal crosslinkingagent). If the internal crosslinking agent is used in less than 0.001mol % or more than 2 mol %, the resultant absorption properties may beinsufficient: there may be a large amount of water-soluble component orthe water absorbency may be too low.

To introduce a crosslinked structure, inside the polymer using theinternal crosslinking agent, the internal crosslinking agent may beadded to the reaction system, for example, before, during, or after thepolymerization of the monomer. Alternatively, the agent may be addedafter neutralization.

The addition of the internal crosslinking agent “during” thepolymerization means both, intermittent addition in at least one periodduring the polymerization of the monomer into the water-absorbent resinand continuous addition throughout the polymerization.

The polymerization of the monomer for the purpose of producing thewater-absorbent resin of the present embodiment may be bulkpolymerization or precipitation polymerization. However, aqueouspolymerization or reverse phase suspension polymerization, in both ofwhich the monomer is used in the form of aqueous solution, is preferredin view of performance, easy controllability of polymerization, and theabsorption properties of swollen gel. The water-absorbent resin obtainedby aqueous polymerization is likely to become colored than thewater-absorbent resin obtained by reverse phase suspensionpolymerization. Effects of the present invention are move evident when,aqueous polymerization is employed.

When the monomer is used in the form of aqueous solution, theconcentration of the monomer in the aqueous solution (hereinafter,“aqueous solution of monomer”) is not limited in any particular mannerand may be determined depending on the temperature of the aqueoussolution, the type of the monomer, and/or other factors. Theconcentration is preferably from 10 to 70 mass %, more preferably from20 to 60 mass %. In the polymerization of the aqueous solution, asolvent(s) other than water may be, used together where necessary. Thetype of solvent used, together is not limited in any particular manner.

In reverse phase suspension polymerization, the aqueous solution ofmonomer is suspended in a hydrophobic organic solvent. The method isdescribed in U.S. patents, such as U.S. Pat. No. 4,093,776,Specification; U.S. Pat. No. 4,367,323, Specification; U.S. Pat. No.4,446,261, Specification; U.S. Pat. No. 4,683,274, Specification; andU.S. Pat. No. 5,244,735, Specification. In aqueous polymerization, nodispersion solvent is used in the polymerization of the aqueous solutionof monomer. The method is described in U.S. patents, such as U.S. Pat.No. 4,625,001, Specification; U.S. Pat. No. 4,873,299, Specification;U.S. Pat. No. 4,286,082, Specification; U.S. Pat. No. 4,973,632,Specification; U.S. Pat. No. 4,985,518, Specification; U.S. Pat. No.5,124,416, Specification; U.S. Pat. No. 5,250,640, Specification; U.S.Pat. No. 5,264,495, Specification; U.S. Pat. No. 5,145,906,Specification; U.S. Pat. No. 5,380,808, Specification; U.S. Pat. No.6,174,978, Specification; U.S. Pat. No. 6,194,531, Specification; U.S.Pat. No. 6,207,772, Specification; and U.S. Pat. No. 6,241,928,Specification. In the present embodiment, the monomers, initiators, etc.given as examples in these U.S. patents which describe reverse phasesuspension polymerization and aqueous polymerization can also be used.

To initiate the polymerization, a radical polymerization initiatorand/or an optical polymerization initiator may be used. Examples of theformer-include potassium persulfate, ammonium persulfate, sodiumpersulfate, t-butylhydroperoxide, hydrogen peroxide, and2,2′-azobis(2-amidino propane) dihydrochloride. An example of the latteris 2-hydroxy-2-methyl-1-phenyl-propane-1-one. Considering the physicalproperties of the water-absorbent resin after the polymerization, thesepolymerization initiators is typically used in 0.001 to 2 mol %,preferably, 0.01 to 0.1 mol % (to the whole monomer).

After the polymerization, the polymer is usually a crosslinked polymerin the form of water-containing gel. The polymer is dried and pulverizedin an ordinary manner before and/or after the drying as required toobtain the water-absorbent resin. Typically, the drying is done at 60 to250° C., preferably at 100 to 220° C., more preferably at 120 to 0.200°C. The drying time depends on the surface area of the polymer, its watercontent ratio (determined in terms of the water content of thewater-absorbent resin and the water-absorbent resin composition andmeasured as reduction in weight before and after drying at 105° C. for 3hours), and the type of the drier. The drying time is set so as toachieve a target water content ratio. Centrifuge retention capacity(CRC) after the drying can be controlled to 20 g/g or greater,preferably 30 to 60 g/g, by tailoring polymerization conditions forwater-absorbent resin, such, as conditions for internal crosslinking, ordrying temperature.

The water-absorbent resin of the present embodiment is preferably of aparticulate shape. The water-absorbent resin is not limited in anyparticular manner in terms of particle size, for example, mass-averageparticle diameter (D50), because the size can be adjusted byclassification or another method after the water-absorbent resincomposition is formed. The mass-average particle diameter (D50) ishowever preferably from 250 to 600 μm, more preferably from 350 to 550μm, even more preferably from 400 to 500 μm, because the diameter inthese ranges requires less labor after the formation of thewater-absorbent resin composition. Particles with particle diametersfrom 150 to 850 μm as obtained with a standard sieve preferably accountfor 90 to 100 mass %, more preferably 95 to 100 mass %, and even morepreferably 99 to 100 mass % of the whole water-absorbent resin.

If reverse phase suspension polymerization is employed in manufacture,the polymer in particulate form may be subjected to dispersionpolymerization and dispersion drying to adjust the particle sizes of thewater-absorbent resin to achieve the range.

If aqueous polymerization is employed, pulverization and classificationmay be carried out following the drying to adjust the particle sizes. Inthe pulverization and classification, the mass-average particle diameterD50 and the rate of particles with diameters less than 150 μm, which aretradeoff conditions, are balanced to achieve a particular particle sizedistribution. For example, to reduce fine particles (<150 μm) whilemaintaining the mass-average particle diameter D50 as low as 600 μm orless, large particles and fine particles may be removed using a sieve orother common classification device after the pulverization if necessary.The large particles removed here are preferably those with diametersfrom 850 μm to 5,000 μm, more preferably from 850 μm to 2,000 μm, evenmore preferably from 850 μm to 1,000 μm. The fine particles removed inthe particle size regulation are preferably those with diameters lessthan 200 μm, more preferably less than 150 μm.

The large particles removed could be discarded, Generally, however, theyare recycled and brought back to another pulverization step forpulverization. The fine particles removed could be discarded.Aggregation of the fine particles (detailed later) however can reduce afall in yield ratio.

The water-absorbent resin prepared by the pulverization step forparticular particle size distribution has an irregularly pulverizedshape.

The fine particles removed in pulverization, classification, or anotherparticle size control step may be recycled to produce large particles ora particulate aggregate for use as the water-absorbent resin of thepresent embodiment. The fine particles can be recycled to produce largeparticles or a particulate aggregate by the methods described in, forexample, U.S. Pat. No. 6,228,930, U.S. Pat. No. 5,264,495, U.S. Pat. No.4,950,692, U.S. Pat. No. 5,478,879, and EP Pat. 844,270. According tothe methods, fine particles and an aqueous liquid, isolated from eachother, are brought into contact with each other under particularconditions to improve viscosity so that the particles can aggregate. Thewater-absorbent resin recovered in this manner practically has a porousstructure.

The water-absorbent resin recovered from the large and fine particlesaccounts for preferably 0 to 50 mass %, more preferably 5 to 40 mass %,and most preferably 10 to 30 mass % of the water-absorbent resin of thepresent embodiment.

When the fine particles in the recovered water-absorbent resin are usedas the water-absorbent resin in accordance with the present invention,those fine particles have a greater surface area than primary particlesof the same particle diameters. Water is quickly absorbed. Thus, theyare better in performance. As explained here, the water-absorbent resinrecovered through aggregation of fine particles are typically mixed withthe water-absorbent resin obtained in the drying step beforepulverization, classification- and particle size control.

In the present embodiment, “particles less than 150 μm” refers toparticles which have passed through a JIS-standard sieve (JIS Z8801-1(2000)) or its equivalent which has 150 μm openings (equal to the sizeto be measured) after classification using a sieve(s) as will bedetailed later. “Particles more than or equal to 150 μm” referssimilarly to particles which have not passed through, and have remainedon, a JIS-standard sieve which has 150 μm openings (equal to the size tobe measured) after classification as will be detailed later. The sameapplies to openings of other sizes. Classification of 50 mass % ofparticles with a mesh having 150 μm openings provides a mass-averageparticle diameter (D50) of 150 μm.

The water-absorbent resin in the water-absorbent resin composition inaccordance with the present invention, which are polymerized throughcrosslinking and dried as above, may be crosslinked further at thesurface (secondary crosslinking) for a modified, polymerizedwater-absorbent resin with improved physical properties.

Various surface crosslinking agents are available for the surfacecrosslinking. Usually, one or two of the following compounds is used inconsideration of physical properties: an oxazoline compound (U.S. Pat.No. 6,297,319), a vinyl ether compound (U.S. Pat. No. 6,372,852), anepoxy compound (U.S. Pat. No. 6,254,990), an oxetane compound (U.S. Pat.No. 6,809,158), a multivalent alcohol compound (U.S. Pat. No.4,734,478), a polyamide polyamine-epihalo adduct (U.S. Pat. No.4,755,562 and U.S. Pat. No. 4,824,901), a hydroxy acrylamide compound(U.S. Pat. No. 6,239,230), an oxazolidinone compound (U.S. Pat. No.6,559,239), a bis- or poly-oxazolidinone compound U.S. Pat. No.6,472,478), a 2-oxotetrahydro-1,3-oxazolidine compound (U.S. Pat. No.6,657,015), and an alkylene carbonate compound (U.S. Pat. No.5,672,633). An alkali (U.S. Pat. No. 2004/106745) or an organicacid/inorganic acid (U.S. Pat. No. 5,610,208) may be used together withthe surface crosslinking agent(s). The monomer may be polymerized at thesurface of the water-absorbent resin for surface crosslinking (U.S. Pat.No. 2005/48221).

Specific examples of the surface crosslinking agent include multivalentalcohol compounds, such as mono, di, tri, tetra, or polyethylene glycol,1,2-propylene glycol, 1,3-propanediol, dipropylene glycol,2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerine,polyglycerine, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol,1,5-pentanediol, 1,6-hexanediol, and 1,2-cyclohexane dimethanol; epoxycompounds, such as ethylene glycol diglycidyl ether and glycidol;multivalent amine compounds, such as ethylenediamine,diethylenetriamine, triethylenetetramine, tetraethyleneperitamine,pentaethylenehexamine, polyethyleneimine, and polyamide polyamine;haloepoxy compounds, such as epichlorohydrin, epibromohydrin, andα-methyl epichlorhydrin; condensates of the multivalent amine compoundsand the haloepoxy compounds; oxazolidinone compounds, such as2-oxazolidinone; oxetane compounds; cyclic urea compounds; and alkylenecarbonate compounds (U.S. Pat. No. 5,409,771), such as ethylenecarbonate. The surface crosslinking agents are however not limited inany particular manner. To further improve the liquid absorptionproperties of the water-absorbent resin, among these surfacecrosslinking agents, it is preferable to use at least one compoundselected from the group consisting of at least oxetane compounds, cyclicurea compounds, and multivalent alcohols, more preferably at least oneselected from the group consisting of oxetane compounds containing 2 to10 carbons and multivalent alcohols, even more preferably a multivalentalcohol having 0.3 to 8 carbons.

The surface crosslinking agent, although depending on the compound(s)used and their combination, may be used in preferably 0.001 mass partsto 10 mass parts, more preferably 0.01 mass parts to 0.5 mass parts, to100 mass part's of the water-absorbent resin.

Water may be used for the surface, crosslinking too. The water, althoughdepending on the water content ratio of the water-absorbent resin used,may be normally used in preferably 0.5 to 20 mass parts, more preferably0.5 to 10 mass parts, to 100 mass parts of the water-absorbent resin. Ahydrophilic organic solvent other than water may be used. The solventmay be used in preferably 0 to 10 mass parts, more preferably 0 to 5mass parts, even more preferably 0 to 3 mass parts, to 100 mass parts ofthe water-absorbent resin.

Preferably, the surface crosslinking agent is mixed with thewater-absorbent resin as follows. Water and/or a hydrophilic organicsolvent are mixed with the surface crosslinking agent in advance ifnecessary. Thereafter, the aqueous solution is sprayed or applieddropwise to the water-absorbent resin (spraying is more desirable). Thespray drops measure preferably 1 to 300 μm, more preferably 10 to 200μm, in average particle diameter. When mixing, water-insoluble fineparticles, a surfactant, etc. may be present so long as their presencedoes not adversely affect effects of the present invention.

The step of surface crosslinking the water-absorbent resin may becarried out either during, or after a step of adding multivalent metal,cations to the water-absorbent resin (detailed later).

The water-absorbent resin mixed with the surface crosslinking agent, ispreferably thermally processed. The process, is carried out, preferably,under the following conditions. Heating temperature (as specified as thetemperature of thermal medium) is preferably 100 to 250° C., morepreferably 150 to 250° C. Heating period is preferably from 1 minute to2 hours. Suitable temperature/period combination examples are 0.1 to 1.5hours at 180° C. and 0.1 to 1 hour at 200° C.

The physical properties (centrifuge retention capacity (CRC), absorbencyagainst pressure (AAP), saline flow conductivity (SFC), which will bedetailed later) of the water-absorbent resin composition arecontrollable through the control of the surface crosslink conditions andparticle size of the water-absorbent resin. Specifically, the values ofAAP, SFC, etc. (detailed later) can be controlled to fall in a preferredrange by reducing the CRC of the water-absorbent resin after the surfacecrosslinking to 0.50 to 0.95 times, preferably 0.60 to 0.90 times, andmore preferably 0.70 to 0.85 times that of the water-absorbent resinbefore the surface crosslinking.

(2) Multivalent Metal Cations

The water-absorbent resin composition of the present embodiment containsmultivalent metal cations.

The inclusion of multivalent metal cations provides further improvedliquid permeability when fabricated into a water-absorbent, resincomposition. The multivalent metal cations inside the water-absorbentresin composition particles make no contribution to liquid permeability.It is therefore preferable to attach multivalent metal cations to thesurface of the water-absorbent resin composition particles. Multivalentmetal cations attach to the surface of the water-absorbent resincomposition particles if multivalent metal cations are added to driedpowder of the water-absorbent resin.

Specifically, a method of manufacturing a water-absorbent resincomposition of the present embodiment involves the steps of:polymerizing an acid group-containing unsaturated monomer into apolycarboxylate-based water-absorbent resin with a crosslinkedstructure, the monomer including an acrylic acid and/or a salt thereofas primary components; and adding multivalent metal cations to thewater-absorbent resin in 0.001 to 5 mass % to the water-absorbent resin,the ratio of Fe cations to the multivalent metal cations other than theFe cations being less than or equal to 0.50 mass %.

The multivalent metal providing the multivalent metal cations is by nomeans limited in any particular manner. A preferred example of the metalis at least one kind of metal atoms selected from the group consistingof Al, Ti, Hf, Zr, and other miscellaneous transition metals. Aparticularly preferred example among them is at least one kind of metalatoms selected from the group consisting of Al, Ti, Hf, and Zr, whichhave strong bonds with carboxyl groups. Al, Zr are further preferredamong them.

In the water-absorbent resin composition in accordance with the presentinvention, the multivalent metal cations other than Fe cations accountfor 6.001 to 5 mass %, preferably 0.001 to 3 mass %, more preferably0.01 to 3 mass %, still more preferably 0.03 to 3 mass %, yet morepreferably 0.04 to 3 mass %, even more preferably 0.1 to 3 mass %, yetmore preferably 0.3 to 2.5 mass %, and most preferably 0.5 to 2 mass %,of the water-absorbent resin. If the multivalent metal cations otherthan Fe cations account for more than 5 mass % of the water-absorbentresin, the water-absorption ratio of the water-absorbent resincomposition may drop. If they account for less than 0.001 mass %,sufficient liquid permeability cannot be imparted to the water-absorbentresin composition. Both cases are undesirable. Multivalent metal cationscontaining such a small amount of Fe cations can be prepared by removingFe cations with an ion adsorbing agent or by another suitably selectedrefining method. If the refining method does not achieve sufficienteffect, the method may be repeated until the multivalent metal cationscontains a small amount of Fe cations.

In the water-absorbent resin composition in accordance with the presentinvention, the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is 0 to 0.50 mass %, preferably 1 ppm to 0.30mass %, even more preferably 10 ppm to 0.20 mass %. If the Fe cations isin an amount in excess of 0.5 mass % to the multivalent metal cationsother than the Fe cations, the Fe cations are likely to react with thewater-absorbent resin in the water-absorbent resin composition, whichwill cause following undesirable phenomena. Degradation of thewater-absorbent resin, such as breakoffs in the polymer chain, arelikely to occur over time or in relation to another factor. In addition,the water-absorbent resin or the water-absorbent resin compositioncolors more often.

For similar reasons, the Fe cations account for preferably 0 to 100 ppm,more preferably 0 to 50 ppm (or 0.01 to 50 ppm), even more preferably 0to 20 ppm (or 0.01 to 20 ppm), still more preferably 0 to 10 ppm, yetmore preferably 0 to 5 ppm, even more preferably 0 to 1, ppm, still morepreferably 0 to 0.5 ppm, yet more preferably 0 to 0.1 ppm, furtherpreferably 0 to 0.01 ppm, and the most preferably less than or equal todetectable limit, to the water-absorbent resin (or water-absorbent resincomposition). It is preferable if the Fe cations in the water-absorbentresin, as well as those in the multivalent metal cations, arecontrolled. The Fe cation content, in the water-absorbent resin to orbelow a particular level can be sufficiently restricted by controllingthe Fe cation content in the material from which the water-absorbentresin is made (for example, aqueous solution of monomer) and the elutionof Fe cations from the manufacturing device (for example, stainlesssteel surface).

The Fe cation and non-Fe cation contents in the multivalent metalcations can be calculated, for example, from the amount of themultivalent metal cation salt added. The contents can also obtained byextracting multivalent metal cations from the water-absorbent resincomposition. Examples of the extraction method include the techniquewhich will be described in examples of the invention later (plasmaemission spectrometry (ICP quantification after extraction in water))and the method described in International application published underthe patent cooperation treaty (PCT) 04/113452.

The multivalent metal cations can be any cations if used as awater-soluble compound. The cations are preferably used as at least onekind of compounds selected from the group consisting of: inorganiccompounds containing OFT, CO₃ ²⁻, or SO₄ ²⁻; organic acid salts, such asacetic acid salt and propionic acid salt; and halides. Preferredexamples of such compounds include aluminum sulfate (includinghydrates)/potassium aluminum sulfate, sodium aluminum sulfate, aluminumhydroxide, an acetylacetone zirconium complex, zirconium acetate,zirconium propionate, sulfate zirconium, zirconium potassiumhexafluoride, zirconium sodium hexafluoride, zirconium ammoniumcarbonate, zirconium potassium carbonate, and zirconium sodium,carbonate. Among them, water-soluble compounds are more preferred.

The multivalent metal cations may be added before the surfacecrosslinking of the water-absorbent resin, simultaneously with thesurface crosslinking, or after the surface crosslinking of thewater-absorbent resin. Preferably, the cations are added to thewater-absorbent resin before or after the surface crosslinking.Especially preferably, the cations are added after the surfacecrosslinking.

The multivalent metal cations may be added in the form of powder (powderparticles) or slurry dispersed in water, an organic solvent, etc.However, the cations are preferably added, in the form of solution whosesolvent is either water or a mixed solvent (water plus organic solvent).Any organic solvent can be used here. Preferred examples includemonovalent alcohols, such as isopropyl alcohol; multivalent alcohols,such as propylene glycol and glycerine; organic acids, such as aceticacid and lactic acid; and organic solvents which mixes well with water,such as acetone and tetrahydrofuran. The solution of the multivalentmetal cations, may contain metal compounds with a valence number lessthan 2, such as sodium hydroxide, sodium carbonate, hydrogen sodiumcarbonate, acetic acid sodium, lactic acid sodium, potassium hydroxide,and lithium hydroxide.

Another trivalent cation or polycation with a greater valence numberthan 3, as well as the multivalent metal cations, may be used to surfacecrosslink the water-absorbent resin. An example is polymer polyamine.

Polymer polyamine is an amine compound which contains three or morecationic groups per molecule. The trivalent cation and polycation with agreater valence number than 3 preferably dissolves in water. A substanceis said to “dissolve in water” (water soluble) if at least 0.5 g,preferably at least 1 g, of it dissolves in 100 g of water at 25° C.

Examples of the trivalent cation and polycation with a greater valencenumber than 3 include cationic polymers, such as polyethyleneimine,polyallylamine, and polyvinyl amine. The cationic polymer has aweight-average molecular weight of preferably 1,000 to 1,000,000, morepreferably 10,000 to 500,000. The cationic polymer is used, for example,in preferably 0 to 10 mass parts, more preferably 0.001 to 8 mass parts,even more preferably 0.01 to 0.5 mass parts, to 100 mass parts of thewater-absorbent resin composition, although the desirable amount variesdepending on the combination of the water-absorbent resin and/or thewater-absorbent resin composition.

(3). Chelating Agent

The water-absorbent resin composition of the present embodimentpreferably further contains a chelating, agent. The use of a chelatingagent restricts the reaction of the Fe cations with the water-absorbentresin because the chelating agent chelates the Fe cations in thewater-absorbent resin composition. That limits the degradation in liquidpermeability of the water-absorbent resin composition which occurs overtime or in relation to another factor.

The chelating agent may be of any kind so long as it can chelate the Fecations. An example is an amino carboxylate-based chelating agent.

The amino carboxylate-based chelating agent is preferably aminocarboxylate and can be ethylenediamine tetracetic acid, hydroxyethylethylenediamine triacetic acid, diethylenetriamine pentacetic acid,triethylene tetramine hexacetic acid, cyclohexane diamino tetraceticacid, methyl glycine diacetic amid, alkali metal salts of these acids,ammonium salts thereof, and amine salts thereof. Especially preferredamong them are amino carboxylates selected from diethylenetriaminepentacetic acid, triethylene tetramine hexacetic acid, methyl glycinediacetic amid, and alkali metal salts of these acids.

The Fe cation content in the water-absorbent resin compositioncontaining the chelating agent is less than or equal to 50 mass %,preferably 200 to 5,000 ppm, more preferably 200 to 1,000 ppm, to themultivalent metal cations other than the Fe cations.

The chelating agent may be used, in any amount. To efficiently chelatethe Fe cations in the water-absorbent resin composition, the chelatingagent content is, for example, preferably 1 ppm to 5 mass %, morepreferably 10; to 1,000 ppm, even more preferably 20 to 200 ppm, to thewater-absorbent resin.

The method of manufacturing a water-absorbent resin composition of thepresent embodiment preferably further involves the step of adding achelating agent to the water-absorbent resin or to the water-absorbentresin composition, wherein the step is carried out either in or after(inclusive of “simultaneous”) the step of polymerizing into thewater-absorbent resin.

The chelating agent may be added to the water-absorbent resin eitherduring or after the step of polymerizing into the water-absorbent resin.Alternatively, the chelating agent may be added simultaneously with thesurface, crosslinking of the water-absorbent resin or after the surfacecrosslinking of the water-absorbent resin.

The addition of the water-absorbent resin “during” the polymerizationmeans both intermittent adding in at least one period during thepolymerization into the water-absorbent resin and continuous addition ofthe water-absorbent resin throughout the polymerization.

(4) Water-Absorbent Resin Composition

The water-absorbent resin composition of the present embodimentmanufactured by the method of manufacturing described above as anexample is a new water-absorbent resin composition (particulate waterabsorption agent).

The water-absorbent resin composition of the present invention is awater-absorbent resin composition (water absorption agent) whichincludes: a polycarboxylate-based water-absorbent resin as a primarycomponent, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the composition is preferably of aparticulate shape, that is, a particulate water absorption agent(particulate water-absorbent resin composition). The water-absorbentresin as a primary component must account for at least 50 mass % andless than 100 mass %, preferably at least 70 mass %, more preferably atleast 80 mass %, even more preferably at least 90 mass % of the entiremass, Besides the water-absorbent resin, the composition contains non-Femultivalent metal cations in an amount of preferably 0.001 to 1 mass %to the water-absorbent resin. The water-absorbent resin compositionfurther contains water (detailed later).

The water-absorbent resin composition of the present embodiment, asmentioned earlier, can be manufactured by, for example, adding themultivalent metal, cations to the water-absorbent resin. Furthermore, itis preferable if the water-absorbent resin composition (particulatewater absorption agent) is specified in terms of the physical propertiesbelow to deliver an excellent absorbent article (hygienic material). Thephysical properties below are controlled so that they fall in apreferred value range, by specifying the multivalent metal cationcontent and the Fe cation content in the water-absorbent resincomposition.

(a) Degraded Liquid Permeation Rate

The water-absorbent resin composition preferably has a degraded liquidpermeation rate (will be defined in an example of the invention) greaterthan 0 seconds and less than or equal to 40 seconds. The degraded liquidpermeation rate is an indicator of the liquid permeability of awater-absorbent resin composition which has changed over time. Thesmaller the value, the higher the liquid permeability. The degradedliquid permeation rate can be made to fall in the above value range by,for example, specifying the particle size of the water-absorbent resincomposition, conditions for surface crosslinking, and the Fe cationcontent and the multivalent metal cation content in the water-absorbentresin composition.

Usually, liquid permeability is measured using, physiological saline, asthe test solution (see, for example, U.S. Pat. No. 6,620,889,Specification; U.S. Pat. No. 6,605,673, Specification; and U.S. Pat. No.6,863,978, Specification). A liquid permeability test usingphysiological saline as the test solution, however, poorly reflects theliquid permeability in actual use with respect to urine. The fact hasprompted the inventors to review the test, which has led to a findingthat the test produces results that are highly related to the liquidpermeability in actual use with respect to urine if the test solutioncontains L-ascorbic acid. By specifying the degraded liquid permeationrate with this new test scheme involving the use of a test solutioncontaining L-ascorbic acid, one can obtain a water-absorbent resincomposition from which a urine-absorbent article (diaper) with excellentliquid permeability in actual use can be made.

Details of the test scheme will be given later in examples of theinvention. The degraded liquid permeation rate of the water-absorbentresin composition is preferably 1 to 35 seconds and more preferably 2 to32 seconds.

When the water-absorbent resin composition is used as the absorbentsubstance in an absorbent article, such as a paper diaper, if thedegraded liquid permeation rate, is in excess of 40 seconds, liquid maynot diffuse in the absorbent substance, possibly causing liquidblocking. The liquid may leak and cause skin rashes and other problemsin the actual use of the paper diaper.

(b) Degraded Soluble Component

The water-absorbent resin composition preferably contains a 0 to 30 mass% degraded soluble component. The degraded soluble component is anindicator of the amount of a water-absorbent resin composition which haschanged over time dissolved in physiological saline containingL-ascorbic acid. Details of the test scheme will be given later inexamples of the invention.

Usually, the soluble component is measured using physiological saline asthe test solution. Such a test, however, poorly reflects the liquidpermeability in actual use with respect to urine. The fact has promptedthe inventors to review the test, which has led to a finding that thetest produces results that are highly related to the liquid permeabilityin actual use with respect to urine if the test solution containsL-ascorbic acid, as has been the case with the degraded liquidpermeation rate. By specifying the degraded soluble component with thisnew test scheme involving the use of a test solution containingL-ascorbic acid, one can obtain a water-absorbent resin composition fromwhich a urine-absorbent article (diaper) with excellent liquidpermeability in actual use cart be made.

The degraded soluble component can be made to fall in the above Valuerange by, for example, specifying crosslink conditions for thepolymerization into the water-absorbent resin, the particle size of thewater-absorbent resin composition, conditions for the surfacecrosslinking, and the Fe cation content and the multivalent metal cationaccount in the water-absorbent resin composition.

The degraded soluble component in the water-absorbent resin compositionis preferably 1 to 28 mass % and more preferably 2 to 25 mass %. If thedegraded soluble component is in excess of 30 mass %, thewater-absorbent resin containing such a water-absorbent resincomposition is likely to develop breakoffs in the polymer chain andother degradation. The water-absorbent resin composition cannot retain asufficient level of liquid permeability over time. Therefore, when thewater-absorbent resin composition is used as the absorbent substance inan absorbent article, such as a paper diaper, liquid may not diffuse inthe absorbent substance, possibly causing liquid blocking. The liquidmay leak and cause skin rashes and other problems in actual use of thepaper diaper.

(c) Centrifuge Retention Capacity (CRC).

The water-absorbent resin composition preferably has a centrifugeretention capacity (CRC) of at least 15 g/g, more preferably 20 to 60g/g, more preferably 25 to 50 g/g, even more preferably 30 to 40 g/g. Ifthe CRC is less than 15 g/g, the composition may not provide sufficientabsorption in actual use. If the CRC is too high (for example, in excessof 50 g/g), the composition gives poor benefits in actual use in view ofthe high manufacture cost and could make it difficult to render anotherphysical property (for example, degrade liquid permeability) fall in apreferred value range. The CRC is, for example, can be made to fall inthe above value range by, for example, specifying conditions for theinternal crosslinking of the water-absorbent resin, conditions for thesurface crosslinking agent, and the Fe cation content and themultivalent metal cation content in the water-absorbent resincomposition.

(d) Absorbency Against Pressure (AAP)

The water-absorbent resin composition preferably has an absorbencyagainst pressure (AAP) of at least 15 g/g. The AAP is the absorptionratio of a water-absorbent resin composition under pressure (4.83 kPa,or about 0.7 psi). Details of the test scheme will be given later inexamples of the invention.

The AAP of the water-absorbent resin composition is preferably at least15 g/g, more preferably 20 to 40 g/g, even more preferably 22 to 30 g/g,most preferably 24 to 29 g/g. If the AAP of the water-absorbent resincomposition is less than 15 g/g, the composition allows less liquiddiffusion and absorbs less liquid under load (e.g., weight of the user).When the water-absorbent resin composition is used as the absorbentsubstance in an absorbent article, such as a paper diaper, the liquidmay not diffuse in the absorbent article, possibly causing liquidblocking. The liquid may leak and cause skin rashes and other problemsin actual use of the paper diaper.

If the AAP of the water-absorbent resin composition is too high (forexample, in excess of 30 g/g), the composition gives poor benefits inactual use in view of high manufacturing cost and could make itdifficult to obtain other physical properties. The AAP of thewater-absorbent resin composition can be made to fall in the above valuerange by, for example, specifying the particle size of thewater-absorbent resin composition, the type and amount of the surfacecrosslinking agent, the Fe cation content and the multivalent metalcation content in the water-absorbent resin composition.

(e) Coloring Value

The water-absorbent resin composition preferably has a coloring value (Lvalue) of more than or equal to 90. The coloring, value (L value) is oneof indicators of coloring. The greater the value, the less the substanceis colored. Details of the test scheme will be given, later in examplesof the invention. The coloring value of the water-absorbent resincomposition can be made to fall in the above range by, for example,specifying the type of the monomer and temperature conditions for thepolymerization of the monomer into the water-absorbent resin, and the Fecation content, and the multivalent metal cation content in thewater-absorbent resin composition.

Water-absorbent resin compositions with improved liquid permeability arenormally used at a high concentration (under a large weight) in diapersand like absorbent articles. If the coloring value (L value) is lessthan 90, when the water-absorbent resin composition is used as theabsorbent substance in an absorbent article, such as a paper diaper, theabsorbent article has a poor appearance, which reduces its commercialvalue.

(f) Shape

The water-absorbent resin composition may have a shape resembling fiber,sheet, or film. The composition however preferably has a particulateshape (particulate water absorption agent). More preferably, thecomposition has a particular particle size for higher liquidpermeability. Specifically, the mass-average particle diameter (D50) ispreferably 250 to 600 μm, more preferably 350 to 550 μm, and even morepreferably 400 to 500 μm. Such particle sizes, of the water-absorbentresin composition are achieved by, for example, pulverization,classification, granulation, and some other particle size controls:

The particles sized 150 to 850 μm account for preferably 90 to 100 mass%, more preferably 95 to 100 mass %, more preferably 99 to 100 mass %,of the entire mass (whole water-absorbent resin composition).

The logarithmic standard deviation (o) of the particle size distributionis 0.10 to 0.50, preferably 0.20 to 0.40, more preferably 0.25 to 0.35.The bulk specific density thereof is preferably 0.50 to 0.82 g/cm², morepreferably 0.55 to 0.78 g/cm², and even more preferably 0.60 to 0.75g/cm².

If the water-absorbent resin composition has a particle size out of thatvalue range, the composition contains a large amount of fine particlesand large particles. The intended performance of a diaper (diaperperformance) may not be achieved in actual use.

(g) Saline Flow Conductivity (SFC)

The water-absorbent resin composition has a saline flow conductivity(SFC) of preferably at least 1 (units: ×10 ⁻⁷·cm³·s·g⁻¹), morepreferably at least 5 (units: ×10 ⁻⁷·cm³·s·g⁻¹), more preferably atleast 20 (units: ×10⁻⁷·cm³·s·g⁻¹), even more preferably at least 40(units: ×10⁻⁷·cm³·s·g⁻¹), most preferably at least 60 (units:×10⁻⁷·cm³·s·g⁻¹). If the water-absorbent resin composition has an SFCless than 1 (units: ×10⁻⁷·cm³·s·g⁻¹), the intended performance of adiaper (diaper performance) may not achieved in actual use. The SFC ofthe water-absorbent resin composition can be rendered to fall in theabove value range by, for example, specifying the particle size of thewater-absorbent resin composition, conditions for the surfacecrosslinking, and the Fe cation content and the multivalent metal cationcontent in the water-absorbent resin composition.

(h) Residual Monomer

The residual monomer in the water-absorbent resin composition iscontrolled to preferably about 0 to 500 ppm, more preferably about 0 to300 ppm. The residual monomer in the water-absorbent resin, compositioncan be rendered to fall in the above value range by, for example,adjusting conditions for the polymerization into the water-absorbentresin and drying conditions.

(i) Water Content Ratio

The water-absorbent resin composition of the present invention furthercontains a small amount of water. The water content, ratio of thewater-absorbent resin composition, defined as the mass reduction in3-hour drying at 180° C. per 1 g of the composition, is preferably 0.1to 20 mass %, more preferably 1 to 15 mass %, even more preferably 1.5to 0.10 mass %, still more preferably 2 to 0.6 mass %. If the watercontent ratio of the water-absorbent resin composition is less than 0.1mass %, the water absorption rate may decrease, and the shock resistanceof the composition in the form of powder may degrade. If thewater-absorbent resin composition has a excessively low shockresistance, the water-absorbent resin composition powder may breakduring transport or fabrication (for example, when the powder isincorporated into a diaper), which undesirably degrades the physicalproperties detailed above (degraded liquid permeation rate, AAP, SFC,and shape). If the water-absorbent resin composition has an excessivelyhigh water content ratio, the water-absorbent resin composition has alow water-absorbent resin content, which degrades water absorbency.Thus, if the water-absorbent resin composition has a water content ratioin excess of 20 mass %, the water absorbency of the water-absorbentresin composition drops greatly, which is undesirable.

(5) Other Additives

The water-absorbent resin composition may further contain variousadditives, such as inorganic powder.

Specific examples of the inorganic powder include metal oxide, such assilicon dioxide and titanium oxide; silicic acid (salt), such as naturalzeolite and synthetic zeolite; kaoline; talc; clay; and bentonite.Preferred among them are silicon dioxide and silicic acid (salt) whichhave an average particle diameter of 200 μm or less as measured with aCoulter counter.

If the inorganic powder is solid particles, the powder may be mixed withthe water-absorbent resin by, for example, dry blending or wet blendingin which both the components are particles. However, if both the powderand composition are particles, the water-absorbent resin composition maynot mix uniformly with the inorganic powder or the water-absorbent resincomposition may not sufficiently attach or bond to the inorganic powder.If such a water-absorbent resin composition is used in diapers and likeabsorbent articles, the water-absorbent resin composition and theinorganic powder may separate and segregate during the course ofmanufacture. That undesirably makes it difficult to manufacture diapersand like absorbent articles with uniform performance.

If the inorganic powder is solid particles, the powder is used in, forexample, preferably 0 to 0.5 mass parts, more preferably 0 to 0.3 massparts, even more preferably 0 to 0.11 mass parts, still more preferably0 to 0.05 mass parts, to every 100 mass parts of the water-absorbentresin composition. The amount can vary depending on the combination ofthe water-absorbent resin and/or the water-absorbent resin composition.If the inorganic powder in the form of solid particles are added inexcess of 0.5 mass parts, that undesirably makes it difficult tomanufacture diapers and like absorbent articles with the uniformperformance described above.

During the course of manufacture of the water-absorbent resincomposition in accordance with the present invention, a deodorizingagent, an antibacterial agent, an aromatic, a foaming agent, a pigment,a dye, a plasticizer, an adhesive agent, a surfactant, a fertilizer, ahoxidizing agent, a reducing agent, water, a salt, a germicidal agent,hydrophilic polymer (e.g. polyethylene glycol), paraffin, a hydrophobicpolymer, a thermoplastic resin (e.g., polyethylene and polypropylene), athermosetting resin (e.g., polyester resin and urea resin), etc. may befurther added, where necessary, in such an amount that the liquidabsorption rate (liquid diffusion rate) does not decrease. For example,about 0 to 10 mass parts of these materials may be added to thewater-absorbent resin and/or the water-absorbent resin composition toevery 100 mass parts of the water-absorbent resin and/or thewater-absorbent resin composition.

(6) Usage

The water-absorbent resin composition in accordance with the presentinvention has excellent moisture absorption characteristics. Thecomposition is applicable in wide range of fields for conventionalwater-absorbent resins from agriculture and gardening to water stopperfor cable, civil engineering, construction, and food processing. Thecomposition is also suitable for use as a solidifying agent(absorbent/gelling agent) for urine, feces, and blood because it hasgood liquid permeability which is an essential physical property forabsorbent substances in diapers and like absorbent articles.

Generally, the absorbent substance is molded containing thewater-absorbent resin composition. The absorbent substance contains thewater-absorbent resin composition in an amount (core concentration) ofpreferably 20 to 100 mass %, more preferably 30 to 100 mass %, even morepreferably 30 to 90 mass %, still more preferably 40 to 80 mass %, tothe combined mass of the water-absorbent resin composition and thehydrophilic fiber. If the core concentration is less than 20 mass %, itbecomes difficult to make use of the properties of the water-absorbentresin composition.

A preferred example of use of the absorbent substance produced from thewater-absorbent resin composition in accordance with the presentinvention is an application to a water-absorbent complex which exhibitsanisotropic expansion (the Complex expands in the thickness direction)described as an example in the specification of U.S. Pat. No. 5,853,867.Using the water-absorbent resin composition in accordance with thepresent invention which shows excellent diffusion, the absorbentsubstance not only expands in the thickness direction, but also showsgreatly improved liquid diffusion in horizontal directions (surfacedirections).

The absorbent substance is preferably compression molded to a density of0.06 to 0.50 g/cc and a basic weight of 0.01 to 0.20 g/cm². The fiberbase material used is hydrophilic fiber (for example, pulverized woodpulp), cotton linter, crosslinked cellulose fiber, rayon, cotton, wool,acetate, vinylon, etc. Preferably, these materials are used after beingaerated.

The absorbent article in accordance with the present invention contains,for example, the absorbent substance, a liquid-permeable front sheet,and a liquid-impermeable back sheet. Specific examples of the absorbentarticle are hygienic materials, such as paper diapers for adults forwhich demand is rapidly growing, kid diapers, sanitary napkins, andso-called “incontinence pads.”

A water-absorbent resin composition in accordance with the presentinvention, as described in the foregoing, is a water-absorbent resincomposition (water absorption agent) characterized in that thecomposition contains: a polycarboxylate-based water-absorbent resin as aprimary component, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the multivalent metal cations otherthan Fe cations account for 0.001 to 1 mass % of the water-absorbentresin; and the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is less than or equal to 5.00 mass %.Accordingly, a water-absorbent resin composition (water absorptionagent) is provided which shows only small reduction in liquidpermeability over time or in relation to another factor and limitedcoloring (especially, over time) and which is suitable for use indiapers and like absorbent articles, as well as for other practicalpurposes.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the ratio of the Fe cations to themultivalent metal cations other than the Fe cations is 1 to 3,000 ppm.

The arrangement further lowers the reduction in liquid permeability ofthe water-absorbent resin composition over time or in relation toanother factor and also further limits the coloring (especially, overtime) of the water-absorbent resin composition.

Another water-absorbent resin composition in accordance with the presentinvention is characterized in that the composition contains: apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of ah acidgroup-containing unsaturated monomer; multivalent metal cations; and achelating agent, wherein: the multivalent metal cations other than Fecations account for 0.001 to 1 mass % of the water-absorbent resin; andthe ratio of the Fe cations to the multivalent metal cations other thanthe Fe cations is less than or equal to 50 mass %. Accordingly, awater-absorbent resin composition is provided which shows only smallreduction in liquid permeability over time or in relation to anotherfactor and limited coloring (especially, over time) and which issuitable for use in diapers and like absorbent articles, as well as forother practical purposes.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the ratio of the Fe cations to themultivalent metal cations other than the Fe cations is 200 to 5,000 ppm.

The arrangement further lowers the reduction in liquid permeability ofthe water-absorbent resin composition over time or in relation toanother factor and also further limits the coloring (especially, overtime) of the water-absorbent resin composition.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if: the composition is of a particulateshape and contains; the multivalent metal cations on surfaces thereof;and the composition is surface crosslinked by a surface crosslinkingagent other than the metal cations.

The arrangement introduces a crosslinked structure to thewater-absorbent resin not only by the multivalent metal cations, butalso by another surface crosslinking agent. The synergistic effectfurther raises the liquid permeability of the water-absorbent resincomposition. The arrangement not only raises the liquid permeability ofthe water-absorbent resin composition, but also improves the absorbencyagainst pressure (AAP) of the water-absorbent resin composition.Furthermore, the arrangement restrains degradation of powder fluidity ofthe water-absorbent resin composition, which is likely to occur uponsurface crosslinking. Therefore, with the arrangement, a water-absorbentresin composition is provided which shows only small reduction in liquidpermeability over time or in relation to another factor, excellentabsorbency against pressure, and limited degradation of powder fluidityand which is suitable for use in diapers and like absorbent articles, aswell as for other practical purposes.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the multivalent metal cations arealuminum cations.

The arrangement further raises the liquid permeability of thewater-absorbent resin composition and powder fluidity upon moistureabsorption.

In the water-absorbent resin composition in accordance with the presentinvention; it is preferable if the composition has a degraded liquidpermeation rate of greater than 0 and less than or equal to 40 seconds.

The arrangement enables the water-absorbent resin composition, when usedin diapers and like absorbent articles, to retain sufficient liquidpermeability in actual use over time. The arrangement provides anabsorbent article with limited leakage.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the composition contains a 0 to 30 mass %degraded soluble component.

The arrangement enables the water-absorbent resin composition, when usedin diapers and like absorbent articles, to retain sufficient liquidpermeability in actual use over time. The arrangement provides anabsorbent article with limited leakage.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the composition shows a coloring value (Lvalue) of more than or equal to 90.0.

The arrangement enables the water-absorbent resin composition, when usedin diapers and like absorbent articles, to show limited coloring for anextended period of time during, distribution and long after purchase,not to mention immediately after its manufacture. The arrangementprovides an absorbent article with an excellent commercial value.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if: the composition is of a particulateshape; and the composition has a mass-average particle diameter of 250to 600 μm and contains 90′ to 100 mass % particles that have a particlediameter of 150 to 850 μm.

The arrangement reduces the fine and large particle content of thewater-absorbent resin composition and gives the water-absorbent resincomposition a particle size suitable for liquid permeability. Thearrangement enables the water-absorbent resin composition, when used indiapers and like absorbent articles, to retain sufficient liquidpermeability in actual use over time. The arrangement provides anabsorbent article with limited leakage.

In the water-absorbent resin composition in accordance with the presentinvention, it is preferable if the acid group-containing unsaturatedmonomer contains 10 to 180 ppm methoxyphenol,

The arrangement enables the resultant, water-absorbent resin to showfurther limited coloring (less yellowish or less yellowing).

A method of manufacturing a water-absorbent resin composition inaccordance with the present invention is characterized in that themethod involves the steps of: (a) polymerizing an acid group-containingunsaturated monomer into a polycarboxylate-based water-absorbent resinwith a crosslinked structure; and (b) adding multivalent metal cationsto the water-absorbent resin in 0.001 to 5 mass % to the water-absorbentresin, the ratio of Fe cations to the multivalent metal cations otherthan the Fe cations being less than or equal to 0.50 mass %.Accordingly, a water-absorbent resin composition is manufactured whichshows only small reduction in liquid permeability over time or inrelation to another factor and which is suitable for use in diapers andlike absorbent articles, as well as for other practical purposes.

In the method of manufacturing a water-absorbent resin, composition inaccordance with the present invention, it is preferable if the methodfurther involves the step of (c) surface crosslinking thewater-absorbent, resin by a surface crosslinking agent other than themultivalent metal cations, step (c) being different from step (b),wherein step (b) is carried out either in or after step (c).

The manufacturing method introduces a crosslinked structure to thewater-absorbent resin not only by the multivalent metal cations, butalso by another surface crosslinking agent. The synergistic, effectfurther raises the liquid permeability of the water-absorbent resincomposition. The method not only raises the liquid permeability of thewater-absorbent resin composition, but also improves the absorbencyagainst pressure (AAP) of the water-absorbent resin composition.Furthermore, the method restrains the degradation of powder fluidity ofthe water-absorbent resin composition, which is likely to occur uponsurface crosslinking. Therefore, with the method, a water-absorbentresin composition is manufactured which shows only small reduction inliquid permeability over time or in relation to another factor,excellent absorbency against pressure, and limited degradation of powderfluidity and which is suitable for use in diapers and like absorbentarticles, as well as for other practical purposes.

In the method of manufacturing a water-absorbent resin composition inaccordance with the present invention, it is preferable if the methodfurther involves the step of (d) adding a chelating agent to thewater-absorbent resin, wherein step (d) is carried out either in orafter step (a).

The manufacturing method eliminates the need to refine or otherwisepurify the multivalent metal cations before use. With the method, awater-absorbent resin composition is manufactured which shows only smallreduction in liquid permeability over time or in relation to anotherfactor and which is suitable for use in diapers and like absorbentarticles, as well as for other practical purposes.

In the method of manufacturing a water-absorbent resin composition inaccordance with the present invention, it is preferable if the methodfurther involves the further step of (e) adjusting a methoxyphenolcontent in the acid group-containing unsaturated monomer used in step,(a) to 10 to 180 ppm. If the acid group-containing unsaturated monomeris in a mixture of acrylic acid and sodium acrylate, which is apreferred embodiment, the methoxyphenol content is obtained as anunneutralized acrylic acid equivalent.

The arrangement enables the resultant water-absorbent resin to showfurther limited coloring (less yellowish or less yellowing).

An absorbent article in accordance with the present invention ischaracterized in that the article is at least one absorbent articleselected from the group consisting of a paper diaper, a sanitary napkin,and an incontinence pad and contains a water-absorbent resin compositionin accordance, with the present invention. Accordingly, an absorbentarticle is provided which shows only small reduction in liquidpermeability over time or in relation to another factor and limitedcoloring (especially, over time).

EXAMPLES

The following will describe the present invention in more detail by wayof examples and comparative examples. The present invention is howeverby no means limited by these examples. The performance of thewater-absorbent resin composition (or water-absorbent resin) wasmeasured by methods discussed below. All electrical devices used in theexamples operated at 200 V or 100 V and 60 Hz. Water-absorbent resincompositions and water-absorbent resins were used at 25° C.±2° C. andrelative humidity of 50% RH, unless otherwise stated. A 0.90 mass %aqueous solution of sodium chloride was used as physiological saline.

The reagents and tools mentioned in the description of measurementmethods and examples are mere examples and can be replaced with suitablealternatives.

The water-absorbent resin composition (or water-absorbent resin) is usedas is (in the form available, for example, on the market with watercontent ratio and other factors not being adjusted) when its variousperformances are measured. When a commercially available water-absorbentresin composition (or water-absorbent resin) or a water-absorbent resincomposition (or water-absorbent resin) in a diaper are used, as samplesin the measurement, however, these samples may have absorbed water overtime. Under such circumstances, these samples are first dried to adjustthe water content to 10 mass % or less, preferably to 5±2 mass %.Thereafter, the physical properties of the water-absorbent resincomposition (or water-absorbent resin) specified in the present exampleof the invention can be measured. The drying may be carried out underany conditions, provided that the water-absorbent resin composition (orwater-absorbent resin) is not decomposed or denatured; preferably, thedrying is carried out at or above room temperature and at or below 60°C. under reduced pressure, for example.

Measurement of Coloring Value

Coloring values (L, a, and b values) of the water-absorbent resincomposition obtained in the examples and comparative examples (detailedlater) were measured using a spectral colorimeter manufactured by NipponDenshoku Industries Co., Ltd. (SZ-Σ80 color measuring system).

Specifically, the coloring values were measured by reflectionmeasurement. Powder and a standard round white board No. 2 for paste usewere designated as standards. A sample base (powder/paste sample base;30 mm in inner diameter and 12 mm in height) which was provided with thecolorimeter and the light projection pipe (30 mm in inner diameter) wereused.

The pre-provided sample base was charged with about 6 g of awater-absorbent resin composition (the pre-provided sample base wasabout 60% full). The surface color (L, a, b) were measured using thespectral colorimeter at room temperature (20 to 25° C.) and a humidityof 50% RH.

Subsequently, the sample base was placed, in a thermostatic humidistat(Platinouslucifer, PL-2G, manufactured by Tabai Espec Co., Ltd.)adjusted to 50±1° C. and 90±1% RH and allowed to stand for 30 days.After that, the surface color (L, a, b) were measured using the spectralcolorimeter.

Degraded Soluble Component

200.0 g of physiological saline containing 0.005 mass % L-ascorbic acid(prepared by dissolving 0.05 g of L-ascorbic acid in 999.95 g of 0.9mass % physiological saline) was weighed and placed in a polypropylenecontainer measuring 65 mm in inner diameter and 90 mm in height(capacity 250 cc). The container was charged with 1.0 g of awater-absorbent resin composition (or water-absorbent resin) obtained inan example or a comparative example (detailed later). After being sealedwith an inner lid and an outer lid, the polypropylene container wasplaced in a thermostatic device (Sibata Thermotec SI-450 Incubator) at37° C. and left for 16 hours. L-ascorbic acid is a degraded product of awater-absorbent resin found in urine.

Following that 16 hours, the polypropylene container was taken out. Thecontent was stirred 1 hour with a stirrer chip at 600 rpm. The stirrerchip was a magnetic stirrer measuring 8 mm in diameter and 35 mm inlength. After the stirring, a soluble component was extracted from thewater-absorbent resin composition (or water-absorbent resin). The liquidextract was filtered with a paper filter (“JIS P 3801 No. 2,” AdvantecToyo Kaisha, Ltd.; thickness 0.26 mm; exclusion size 5 μm). 50.0 g ofthe resultant filtered solution was weighed as a measurement solution.

First, a 0.1 N aqueous solution of NaOH was added only to thephysiological saline containing 0.005 mass % L-ascorbic acid to pH 10.Thereafter, a 0.1 N aqueous solution of HCl was added to pH 2.7 todetermine blank titer ([bNaOH] ml, [bHCl]ml).

Similar titration was done for the measurement solution to determinetiter ([NaOH] ml, [HCl] ml).

For example, for a water-absorbent resin containing a known amount of anacrylic acid and sodium salt thereof, the amount of the solublecomponent in the water-absorbent resin can be calculated from theaverage molecular weight of the monomer and the titer determined by thetitration, using the following formula:

Degraded Soluble Component (mass %)=0.1×(Average Molecular Weight ofMonomer)×184.3×100×([HCl]−[bHCl])/1000/1.0/50.0

Neutrality Ratio (mol %)=[1−([NaOH]−[bNaOH])/([HCl]−[bHCl])]×100

If the amount of the acrylic acid and sodium salt thereof was unknown,the average molecular weight of the monomer was calculated from theneutrality ratio determined by the titration.

The average molecular weight of the monomer can be calculated using thefollowing formula:

Average molecular weight of Monomer=Neutrality Ratio (mol%)/100×Molecular Weight of Sodium Acrylate (94.05)+(100−Neutrality Ratio(mol %))/100×Molecular Weight of Acrylic Acid (72.06)

Soluble Component

The same procedures were carried out as the measurement of degradedsoluble component to calculated the soluble component, except thatphysiological saline was used in place of the physiological salinecontaining 0.005 mass % L-ascorbic acid.

Degraded Liquid Permeation Rate

200.0 g of physiological saline containing 0.005 mass % L-ascorbic acid(prepared by dissolving 0.05 g of L-ascorbic acid in 999.95 g of 0.9mass % physiological saline) was weighed and placed in a polypropylenecontainer measuring 65 mm in inner diameter and 90 mm in height(capacity 250 cc). The container was charged with 1.0 g of awater-absorbent resin composition obtained in an example or acomparative example (detailed later). After being sealed with an innerlid and an outer lid, the polypropylene container was placed in athermostatic device (Sibata Thermotec SI-450 Incubator) at 37° C. andleft for 16 hours.

Following that 16 hours, the polypropylene container was taken out. Thedegraded liquid permeation rate was measured using the device shown inFIG. 1. As shown in FIG. 1, the measuring device for the degraded liquidpermeation rate is made up of: a cylindrical acrylic cell (innerdiameter 60 mm; height 50 mm) 1 with a 100-mesh wire net (150 μm) 4attached to one of two openings thereof; a support base 6; and a 20-meshwire net (850 μm) 5 disposed on the support base 6. The acrylic cell 1was placed on the 20-mesh wire net 5 so that a 100-mesh wire net 4 ofthe acrylic cell could be in contact with the 20-mesh wire net 5.

All the water-absorbent resin composition 3 in the polypropylenecontainer removed from the thermostatic device was placed to the acryliccell 1. Water was removed from the top of the gel of the water-absorbentresin composition 3 until there was no liquid being left (to make thetop of the gel flat). After removing water, a glass filter 2 wasdisposed on the water-absorbent resin composition 3 and let to stand for1 minute. Following that 1 minute, 0.9 mass % physiological saline 7(100 g) measured in a 100 mL beaker was poured into the acrylic cell 1.Time was measured from the start of the pouring until there was noliquid left on the top of the glass filter 2. That time measurement wasthe degraded liquid permeation rate (seconds).

Centrifuge Retention Capacity (CRC)

W g (=about 0.20 g) of a water-absorbent resin composition (orwater-absorbent resin) obtained in an example or a comparative example(detailed later) was put in a bag of non-woven fabric (60 mm×85 mm;material complying with EDANA ERT 441.1-99) uniformly and sealed. Thebag was immersed in 0.90 mass % physiological saline of Which thetemperature was regulated at 25±2° C. After 30 minutes, the bag wastaken out and rid of water in a centrifugal separator (CompactCentrifugal Separator H-122, Kokusan Corp.) at 250 G (250×9.81 m/s²) for3 minutes. Then, the mass of the bag was measured (=W2 (g)). A similaroperation was done without using the water-absorbent resin composition(or water-absorbent resin). The mass of the bag was measured again (=W1(g)). CRC (g/g) was calculated from the masses W, W1, W2 using thefollowing formula.

CRC (g/g)={(Mass W2 (g)−Mass W1 (g))/Mass W (g)}−1

Absorbency Against Pressure (AAP)

A 400-mesh stainless steel wire net (openings 0.38 μm) was fused to thebottom of a plastic support cylinder with an inner diameter of 60 mm. W(g) (=about 0.90 g) of a water-absorbent resin composition (orwater-absorbent resin) was placed uniformly on the net. A piston and aload were placed on the net in this order. The piston and load weretailored so that they could place a uniform load of 4.83 kPa (=about 0.7psi) to the water-absorbent resin composition (or water-absorbentresin). The piston and load had an external diameter slightly less than60 mm, formed no gap between them and the support cylinder, and couldmove up and down freely in the cylinder. The mass of the entiremeasuring system, including the measuring device, but minus the loaditself, (the mass of the support cylinder, the water-absorbent resincomposition (or water-absorbent resin), and the piston) was measured(=W3 (g)) before the load was placed.

A glass filter with a diameter of 90 mm and a thickness of 5 mm(available from Sogo Laboratory Glass Works Co., Ltd.; narrow holediameter 100 to 120 μm) was placed on a petri dish with a diameter of150 mm. 0.90 mass % physiological saline was added so that the salinewas flush was the top of the glass filter. A piece of paper filter witha diameter of 90 mm (No. 2, JIS P 3801, Toyo Roshi Kaisha, Ltd.,Advantec) was placed on top of it so that the surface could be all wet.Excess liquid was removed.

The entire measuring system, including the measuring device, was placedon the wet paper filter mentioned above. The water-absorbing resincomposition (or water-absorbing resin) absorbed the liquid under load.If the liquid surface lowered below the top of the glass filter, liquidwas added to maintain the liquid surface at a constant level. After 1hour, the system, including the measuring device, was lifted. The massof the system minus the load (the mass of the support cylinder, theswollen water-absorbent resin composition (or water-absorbent resin),and the piston) was measured again (=W4 (g)). AAP (g/g) was calculatedfrom the masses W, W3, W4 using the following formula.

AAP (g/g)=(Mass W4 (g)−Mass W3 (g))/W (g)

Mass-average Particle Diameter (D50) and Logarithmic Standard Deviation(σζ),

A water-absorbent resin composition (or water-absorbent resin) wassieved using JIS-compliant sieves with openings measuring, for example,850 μm, 710 μm, 600 μm, 500 μm, 300 μm, 150 μm, and 45 μm. The residualpercentages R were plotted on logarithm probability paper. The particlediameter corresponding to R=50 mass % was read from the graph. Thereading was the mass-average particle diameter (D50)). The logarithmicstandard deviation (σζ) is given by;

σζ=0.5×ln(X2/X1)

where X1 and X2 are particle diameters respectively for R=84.1 andR=15.9%. The smaller the σζ, the narrower the particle sizedistribution.

For the measurement of the mass-average particle diameter (D50) and thelogarithmic standard deviation (σζ), the particles were classified asfollows. 10.0 g of the water-absorbent resin composition (orwater-absorbent resin) was placed in JIS-compliant sieves (Iida TestingSieves; diameter 8 cm) with openings measuring 850 μm, 710 μm, 600 μm,500 μm, 300 μm, 150 μm, and 45 μm at room temperature (20 to 25° C.) and50% RH humidity. The sieves were shaken in a vibration classifier (IidaSieve Shaker, ES-65, Ser. No. 0501) for 5 minutes to completeclassification. Basically, the sieves of the types mentioned here wereused; sieves of other types however could also be used if they were,suited for the particle size of the water-absorbent resin.

Saline Flow Conductivity (SFC)

SFC is a value which indicates the permeability of a swollenwater-absorbent resin to liquid. The greater the SFC value, the higherthe liquid permeability.

A saline flow conductivity (SFC) test was conducted as described inPublished Japanese Translation of PCT Application 9-509591/1997(Tokuhyohei 9-50.959).

The following will describe devices used in a SFC test in reference toFIG. 2.

The device shown in FIG. 2 has a glass tube 32 inserted in a container31. The lower end of the glass tube 32 is positioned so as to maintain0.69 mass % physiological saline 33 at a height of 5 cm above the bottomof a swollen gel 37 in a cell 39. The 0.69 mass % physiological saline33 in the container 31 is supplied to the cell 39 via an L-shaped tube34 fitted with a cock 35. Below the cell 39 is there positioned acontainer 48 which collects liquid which has been passed through. Thecontainer 48 sits on pan scales 49. The cell 39 has an inner diameter of6 cm and is fitted on its bottom with a No. 400 stainless steel wire net(openings 38 μm) 38. The piston 46 has, on its lower part, holes 47which sufficiently let liquid pass through them. The piston 46 is fittedon its bottom a high permeability glass filter 45 to prevent thewater-absorbent resin composition (or water-absorbent resin) or itsswollen gel from entering the holes 47. The cell 39 is placed on a basewhich supports the cell 39. A stainless steel wire net 36 is placed onthe surface of the base which is in contact with the cell 39. The net 36does not disrupt liquid transmission.

Using the device shown in FIG. 2, a water-absorbent resin (0.900 g)placed uniformly in the container 40 was let to swell in artificialurine under a pressure of 2.07 kPa (=about 0.3 psi) for 60 minutes. Theheight, of the gel layer in the gel 37 was recorded. Next, the 0.69 mass% physiological saline 33 was passed from the container 31 through theswollen gel layer under a pressure of 2.07 kPa (=about 0.3 psi) whilemaintaining hydrostatic pressure at a constant value. The SFC test wasconducted at room temperature (25° C.±2° C.). The amount of liquidpassed through the gel layer was recorded as a function of time at20-second intervals over 10 minutes, using a computer and the pan scales49. The flow rate Fs (units: g/s) at time T through the swollen gel 37(chiefly between particles) was determined by dividing weight increase(g) by time (s). Suppose that a constant hydrostatic pressure and astable flow rate were achieved at time Ts. Flow rates were calculatedonly from data collected between time Ts and time Ts+10 minutes. Theseflow rates were then used to calculate Fs at T=0, that is, the firstflow rate through the gel layer. Fs at T=0 was calculated byextrapolating Fs(T) by a least square method to T=0. The units of SFCwere 10⁻⁷·cm³·s·g⁻¹.

SFC=(Fs(T=0)×L0)/(ρ×A×ΔP)=(Fs(T=0)×L0)/139506

where

Fs(T=0): Flow rate in g/s

L0: Height of gel layer in cm.

ρ: Density of NaCl solution (=1.003 g/cm³)

A: Area of upper surface of gel layer in cell 41 (=28.27 cm²)

ΔP: Hydrostatic pressure exerted on gel layer (=4920 dyne/cm²)

The artificial urine used in the SFC test contained 0.25 g of dihydrateof calcium chloride, 2.0 g of potassium chloride, 0.50 g of hexahydrateof magnesium chloride, 2.0 g of sodium sulfate, 0.85 g of ammoniumdihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate, and994.25 g of pure water.

Method of Quantification of Multivalent Metal Component inWater-Absorbent Resin (i) Plasma Emission Spectrometry

1.0 g of a water-absorbent resin composition was weighed and placed in apolypropylene beaker with a capacity of 260 mL. 190.0 g of physiologicalsaline (0.9 mass % NaCl aqueous solution) and 10.0 g of 2 N hydrochloricacid were added and stirred 30 minutes with a stirrer chip (magneticstirrer, diameter 8 mm, length 35 mm) at 600 rpm at room temperature.After the stirring, the supernate was filtered by a chromatodisk (GLchromatodisk 25A, GL Sciences Inc.). The filtrate was analyzed by plasmaemission spectrometry (Ultima, Horiba Ltd.) to determine the multivalentmetal component, concentration. A calibration curve was drawn using aphysiological saline containing a known amount of multivalent metalcomponent. In the analysis above, a base line correction was carried outconsidering the composition of the physiological saline. From thatconcentration, the multivalent metal component concentration in thewater-absorbent resin composition was calculated using the formula:

Multivalent Metal Component Concentration in Water-absorbent ResinComposition (mass %)=(Multivalent Metal Component Concentration inSolution (mass %))×200

From the multivalent metal component concentration (mass %) in thewater-absorbent resin composition, the multivalent metal componentconcentration in the water-absorbent resin was determined using thefollowing:

Multivalent Metal Component Concentration in Water-absorbent Resin (mass%)=Multivalent Metal Concentration in Water-absorbent Resin Composition(mass %)/Water-absorbent Resin in Water-absorbent Resin Composition(mass %)×100

(ii) Method of Measurement of Multivalent Metal Component ExtractionRatio

Multivalent metal component extraction ratio reflects the state of amultivalent metal in a water-absorbent resin, that is, whether themultivalent metal has formed salt which, is highly insoluble to water orsalt which is easily soluble to water. For example, in the case, ofaluminum sulfate added to a water-absorbent resin, the ratio reflectswhether the aluminum sulfate is present in such a state that it hassurface crosslinked the water-absorbent resin or as present it is.Specifically, the ratio indicates how much of the aluminum sulfate addedto the water-absorbent resin composition remains as aluminum sulfate.The following will describe how to measure the multivalent metalcomponent extraction ratio.

95 g of a methanol solution of 1.0 mass % 8-quinolinol (obtained fromWako Pure Chemical Ind.) was mixed with 5 g of pure water to preparesolution A. 95 g of methanol was mixed with 5 g of pure water mix toprepare solution B.

A stirrer chip (magnetic stirrer, diameter 8 mm, length 35 mm) wasplaced in a 260-mL polypropylene container. 5 g of a water-absorbentresin composition and 25 g of solution A were weighed and placed in thecontainer. The container was then sealed. The content was stirred for 20hours with the magnetic stirrer at 600 rpm at room temperature. 5 mL ofthe resultant supernate was sucked by a polypropylene syringe. Achromatodisk (GL chromatodisk 25A, GL Sciences Inc.) was attached to thesyringe with which the solution was measured and sucked, to filter thesupernate. Part of the filtrate was transferred to a 1-cm plastic cell(Dispocell, Model No. 2-478-03, Type No. 1939, purchased from AsoneCorp.). Absorption of particular wavelength which occurs due to theformation of a multivalent metal component/8-quinolinol complex wasmeasured by a spectrophotometer (Hitachi Ratio Beam SpectrophotometerU-1100). The particular wavelength is, for example, 380 nm if themultivalent metal component is aluminum. For convenience, the followingwill assume that the multivalent metal component is aluminum and thatthe particular wavelength is 380 nm. If the 380-nm absorption by thefiltrate is beyond the measurable range of the spectrophotometer, thefiltrate is diluted by solution B so that the absorption falls in themeasurable range of the spectrophotometer.

The absorption when 100 mass % of the multivalent metal component wasextracted was obtained by measuring the 380-nm absorption by a solutionin which the multivalent metal compound was so dissolved in solution Athat the same amount of multivalent metal component would be present aswhen 100 mass % of the multivalent metal component was extracted. Themultivalent metal component concentration in the water-absorbent resincomposition was separately obtained by the plasma emission spectrometrydescribed earlier.

The extraction ratio for the multivalent metal component is given, by:

Extraction Ratio for Multivalent Metal Component (mass %)=((Absorbencyof 380 nm by Filtrate)−(Absorbency of 380 rim by SolutionA))/((Absorbency of 380 nm by Multivalent Metal Component Extracted 100mass %) (Absorbency of 380 nm by Solution A))×100

Methoxyphenol Content, Furfural Content

Quantitative analysis was performed on standard samples under thefollowing conditions, using a gas chromatograph (made, by ShimadzuCorporation, GC-7A model) and a data processor (made by ShimazuCorporation, C-R6A model) to obtain the methoxyphenol content and thefurfural content in the monomer (acrylic acid):

Detector: FID

Temperature of Column Thermostat Bath: 200° C.

Temperature of Sample-introducing Section: 250° C.

Column: High Polarity Capillary Column (30 cm in length, 0.5 mm in innerdiameter, and 1.5 μm in thickness).

For the case of compositions of acrylic, acid and sodium acrylate, themethoxyphenol content and the furfural content were obtained as anacrylic acid equivalent.

Reference Example 1

An aqueous solution of sodium acrylate having a neutrality ratio of 71:3mol % (the p-methoxyphenol content in the acrylic acid was 200 ppm; thefurfural content, in the acrylic acid was 0.5 ppm) was prepared from anacrylic acid refined by distilling a commercially available acrylic acid(special grade reagent containing 200 ppm p-methoxyphenol, manufacturedby Wako Pure Chemical Ind.) and a commercially available sodiumhydroxide (special grade reagent manufactured by Wako Pure ChemicalInd.). 4.0 mass parts of polyethylene glycol diacrylate (average numberof ethylene oxide added=8 moles) was dissolved in 5,500 mass parts(monomer concentration=38 mass %) of that aqueous solution of sodiumacrylate, to prepare a reaction solution. The reaction solution wasdeaerated for 30 minutes in a nitrogen gas atmosphere. Next, thereaction solution was placed in a reaction vessel which was a liddedstainless steel double-arm kneader with two sigma-type blades and ajacket (internal volume 10 L). Nitrogen gas was substituted in thereaction vessel while maintaining the reaction solution at 30° C.

Subsequently, 2.8 g of sodium persulfate and 0.01 g of L-ascorbic acidwere added in the form of aqueous solution while stirring the reactionsolution. About 1 minute after the addition, polymerization started.Polymerization was carried out initially at 30° C. and then at 30 to 90°C. 60 minutes into the polymerization, a water-containing gel-likecrosslinked polymer, was removed. The obtained, water-containinggel-like crosslinked polymer was divided into fragments, each measuringabout 1 to 2 mm in diameter. The divided water-containing gel-likepolymer was spread on a 300 μm wire net and dried in hot wind at 150° C.for 90 minutes. The dried product was pulverized in a vibration mill andclassified using a 850-μm-opening wire net for particle size adjustment,to prepare irregularly pulverized water-absorbent resin powder.

100 mass parts of the obtained water-absorbent resin powder was mixedwith an organic surface crosslinking agent containing 0.5 mass partpropylene glycol, 0.03 mass part ethylene glycol diglycidyl ether, 0.3mass part 1,4-butanediol, and 3 mass part water. The mixture was heatedat 200° C. for 40 minutes to yield a water-absorbent resin.

The obtained water-absorbent resin showed a centrifuge retentioncapacity (CRC) of 30 g/g, an absorbency against pressure (AAP) of 25.8g/g, and a saline flow conductivity (SFC) of 35×10⁻⁷·cm³·s·g⁻¹. Besides,the resin showed a mass-average particle diameter (D50) of 420 μm and alogarithmic standard deviation (σζ) of 0.42. The resin containedparticles sized 150 to 850 μm in a proportion of at least 96.9% to theresin composition.

As to the coloring values of the water-absorbent resin, the L value was90.2, the a value was 0.37, the b value was 5.5. The degraded solublecomponent was 25.1%.

Reference Example 2

1.0 mass parts of a 50% aqueous solution of aluminum sulfate (containing10 ppm Fe cations, Asahi Chemical Co., Ltd.); 0.025 mass parts ofpropylene glycol, and 0.167 mass parts of a 60% aqueous solution ofsodium lactate were mixed to prepare preparation solution (a).

Mixed solutions (b-1) to (b-7) were prepared by adding a 10% aqueoussolution of ferrous sulfate to preparation solution (a). Each mixedsolution (b-1) to (b-7) contained 1.192 mass parts of preparationsolution (a). Mixed solutions (b-1) to (b-7) contained, respectively,0.0027 mass parts, 0.027 mass parts, 0.14 mass parts, 0.27 mass parts,0.54 mass parts, 1.4 mass parts, and 2.7 mass parts of the 10% aqueoussolution of ferrous sulfate.

Mixed solutions (c-1) to (c-7) were prepared by further adding a 46 mass% aqueous solution of trisodium diethylenetriamine pentacetate (ChelestPC-45, Chubu Chelest Co., Ltd.) to mixed solutions (b-1) to (b-7). Mixedsolution (c-1) to (c-7) contained, respectively, 0.0007 mass parts,0.007 mass parts, 0.033 mass parts, 0.066 mass parts/0.131 mass parts,0.328 mass parts, and 0.656 mass parts of the 46 mass % aqueous solutionof trisodium diethylenetriamine pentacetate. The molar quantity of thetrisodium diethylenetriamine pentacetate in each mixed solution (c-1) to(c-7) was equivalent to 0.37 times the molar quantity of the Fe cations(ferrous sulfate).

Reference Example 3

A water-absorbent resin composition was prepared by following the sameprocedures as in reference example 1, except that a different reactionsolution was used. The reaction solution was prepared by dissolving 6.0mass parts of polyethylene glycol diacrylate (average number of ethyleneoxide added=8 moles) in 5,500 mass parts of the aqueous solution ofsodium acrylate having a neutrality ratio of 71.3 mol % used inreference example 1.

The obtained water-absorbent resin showed a centrifuge retentioncapacity (CRC) of 27 g/g, an absorbency against pressure (AAP) of 25g/g, and a saline flow conductivity (SFC) of 60×10⁻⁷·cm³·s·g⁻¹. Besides,the resin showed a mass-average particle diameter (D50) of 430 μm and alogarithmic standard deviation (σζ) of 0.41. The resin containedparticles sized 150 to 850 μm in a proportion of at least 97.1% to theresin composition. As to the coloring values of the water-absorbentresin, the L value was 91.0, the a value was −0.35, the b value, was,5.4. The degraded soluble component was 20.1%.

Example 1

1.192 mass parts of preparation solution (a) was added to and mixed with100 mass parts of the water-absorbent resin obtained in referenceexample 1. The mixture was put into vinyl bag fitted with a zipper andlet to cure for 1 hour in a drier device at 60° C. After the curing, themixture was passed through a 20-mesh (850 μm) sieve to yield awater-absorbent resin composition (D-1). The obtained water-absorbentresin composition showed a centrifuge retention capacity (CRC) of 30g/g, an absorbency against pressure (AAP) of 24.2 g/g, and a saline flowconductivity (SFC) of 60×10⁻⁷·cm³·s·g⁻¹. Besides, the resin showed amass-average particle diameter (D50) of 420 μm and a logarithmicstandard deviation (σζ) of 0.42. The resin contained particles sized 150to 850 μm in a proportion of 96.9% to the resin composition.Measurements of the coloring values, degraded soluble component, anddegraded liquid permeation rate of water-absorbent resin composition(D-1) are shown in Table 1.

Examples 2, 3

Water-absorbent resin compositions (D-2), (D-3) were prepared byfollowing the same procedures as in example 1, except that 1.1.95 massparts of mixed solution (b-1) and 1.219 mass parts of mixed solution(b-2), instead of the 1.192 mass parts of preparation solution (a), wereadded respectively to form compositions (D-2), (D-3). Obtainedwater-absorbent resin compositions (D-2), (D-3) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow conductivity (SFC) to those of water-absorbent resincomposition (D-1). Compositions (D-2), (D-3) also showed an equalmass-average particle diameter (D50) and logarithmic standard deviation(o to those of composition (D-1). Compositions (D-2), (D-3) containedparticles sized 150 to 850 μm in the same proportion as composition(D-1). Measurements of the coloring values, degraded soluble component,and degraded liquid permeation rate of water-absorbent resincompositions (D-2), (D-3) are shown Table 1.

Examples 4 to 8

Water-absorbent resin compositions (D-4) to (D-8) were prepared byfollowing the same procedures as in example 1, except that 1.195 massparts of mixed solution (c-1), 1.226 mass parts of mixed solution (c-2),1.361 mass parts of mixed solution (c-3), 1.529 mass parts of mixedsolution (c-4), and 1.866 mass parts of mixed solution (c-5), instead ofthe 1.192 mass parts of preparation solution (a), were addedrespectively to form compositions (D-4) to (D-8). Obtainedwater-absorbent resin compositions (D-4) to (D-8) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow conductivity (SFC) to those of water-absorbent resincomposition (D-1). Compositions (D-4) to (D-8) also showed an equalmass-average particle diameter (D50) and logarithmic standard deviation(σζ) to those of composition (D-1). Compositions (D-4) to (D-8)contained particles sized 150 to 850 μm in the same proportion ascomposition (D-1). Measurements of the coloring values, degraded solublecomponent, and degraded liquid permeation rate of water-absorbent resincompositions (D-4) to (D-8) are shown in Table 1.

Comparative Examples 1 to 5

Water-absorbent resin compositions (D-9) to (D-13) were prepared byfollowing the same procedures as in example 1, except that 1.332 massparts of mixed solution (b-3), 1.462 mass parts of mixed solution (b-4),1.732 mass parts of mixed solution (b-5), 2.592 mass parts of mixedsolution (b-6), and 3.892 mass parts of mixed solution (b-7), instead ofthe 1.192 mass parts of preparation solution (a), were addedrespectively to form compositions (D-9) to (D-13). Obtainedwater-absorbent resin compositions (D-9) to (D-13) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow conductivity (SFC) to those of water-absorbent resincomposition (D-1). Compositions (D-9) to (D-13) also showed an equalmass-average particle diameter (D50) and logarithmic standard deviation(σζ) to those of composition (D-1). Compositions (D-9) to (D-13)contained particles sized 150 to 850 μm in the same proportion ascomposition (D-1). Measurements of the coloring values, degraded solublecomponent, and degraded liquid permeation rate of water-absorbent resincompositions (D-9) to (D-13) are shown in Table 1.

Comparative Examples 6, 7

Water-absorbent resin compositions (D-14), (D-15) were prepared byfollowing the same procedures as in example 1, except that 2.877 massparts of mixed solution (c-6) and 4.563 mass parts of mixed solution(c-7), instead of the 1.192 mass parts of preparation solution (a), wereadded respectively to form compositions (D-14), (D-15). Obtainedwater-absorbent resin compositions (D-14), (D-15) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow, conductivity (SFC) to those of water-absorbent resincomposition (D-1). Compositions (D-14), (D-15) also showed an equalmass-average particle diameter (D50) and logarithmic standard deviation(σζ) to those of composition (D-1). Compositions (D-14), (D-15)contained particles sized 150 to 850 μm in the same proportion ascomposition (D-1). Measurements of the coloring values, degraded solublecomponent, and degraded liquid permeation rate of water-absorbent resincompositions (D-14), (D-15) are shown in Table 1.

Example 9

A commercially available acrylic acid (special grade reagent containing200 ppm p-methoxyphenol, manufactured by Wako Pure Chemical Ind.)prepared by gas phase catalytic oxidation was placed on the bottom of ahigh boiling point impurity separation tower equipped with 50 stagesperforated boards without downcomer, for distillation at a reflux ratioof 1. The distilled acrylic acid was distilled again under the sameconditions. Next, p-methoxyphenol was added to the redistilled acrylicacid to obtain refined acrylic acid. The refined acrylic acid contained90 ppm p-methoxyphenol and furfural in an amount less than or equal tothe detectable limit (1 ppm). The same procedures were carried out as inreference example 1, except that the refined acrylic acid was used inplace of the acrylic acid in reference example 1 (that is, an aqueoussolution of sodium acrylate (monomer concentration 38 mass %) preparedby neutralizing the refined acrylic acid to a neutrality ratio of 71.3mol % was used in place of the acrylic acid in reference example 1), toprepare a water-absorbent resin. Furthermore, the same procedures werecarried out as in example 1 to prepare water-absorbent resin composition(D-16).

Obtained water-absorbent resin composition (D-16) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow conductivity (SFC) to those of water-absorbent, resincomposition (D-1). Composition (D-16) also showed an equal mass-averageparticle diameter (D50) and logarithmic standard deviation (σζ) to thoseof composition (D-1). Composition (D-16) contained particles sized 150to 850 μm in the same proportion as composition (D-1). Measurements ofthe coloring values, degraded soluble component, and degraded liquidpermeation rate of water-absorbent resin composition (D-16) are shown inTable 1.

Comparative Example 8

A water-absorbent resin was prepared by following the same procedures asin example 9, except that a commercially available acrylic acid (specialgrade reagent containing 200 ppm p-methoxyphenol, manufactured by WakoPure Chemical Ind.), instead of the refined acrylic acid, was used asthe acrylic acid. The same procedures were carried out as in comparativeexample 0.7 to prepare water-absorbent resin composition (D-17).

Obtained, water-absorbent resin composition (D-17) showed an equalcentrifuge retention capacity (CRC), absorbency against pressure (AAP),and saline flow conductivity (SFC) to those of water-absorbent resincomposition (D-1). Composition (D-17) also showed an equal mass-averageparticle diameter (D50) and logarithmic standard deviation (σζ) to thoseof composition (D-1). Composition (D-17) contained particles sized 150to 850 μm in the same proportion as composition (D-1). Measurements ofthe coloring values, degraded soluble component, and degrade liquidpermeation rate of water-absorbent resin composition (D-17) are shown inTable 1. Water-absorbent resin composition (D-17) exhibited thefollowing coloring values after being left to stand for 30 days at 50°C. and 90% RH: L=53.2, a=6.6, and b=16.1.

Comparative Example 9

A 2-L polyethylene container was charged with 100 mass parts of thewater-absorbent resin obtained by reverse phase suspensionpolymerization (detailed below) and 2 mass parts of sodium sulfite A(“Sulfurous Anhydride of Soda for Food Additive” containing 1.4 ppmiron, manufactured by Daito Chemical Col, Ltd.). The mixture, was thenmixed for 1 hour in a cross rotary mixer, at a rotation, rate of 30 rpmand revolution rate of 30 rpm to obtain water-absorbent, resincomposition (D-18). Water-absorbent resin composition (D-18) contained0.3 ppm iron.

Manufacture of Water-absorbent Resin by Reverse Phase SuspensionPolymerization: An Example

A five-necked cylindrical round bottom, flask with a capacity of 1000 mLequipped with a stirrer, a reflux cooler, a dropping funnel, athermometer, and a nitrogen gas introduction tube was charged with 500mL of n-heptane 0.92 g of sucrose fatty acid ester (S-370 manufacturedby Mitsubishi Chemical, Co., Ltd.) with a HLB of 3.0 was then added anddispersed as a surfactant to dissolve the surfactant.

Apart from the five-necked cylindrical round bottom flask, a triangleflask with a capacity of 500 mL was charged with 92 g of an 80 mass %aqueous solution of acrylic acid (the acrylic acid was theaforementioned commercially available acrylic acid (special gradereagent containing 200 ppm p-methoxyphenol, manufactured by Wako PureChemical Ind.)). 102.2 g of a 30 mass % aqueous solution of sodiumhydroxide was added dropwise to the triangle flask while the flask wasbeing externally cooled, so as to prepare a partly neutralized productof acrylic acid in Which 75 mol % of the acrylic acid was neutralized.Furthermore, 50.2 g of water, 0.11 g of potassium persulfate as apolymerization initiator, and 9.2 mg of ethylene glycol diglycidyl etheras a crosslink agent were added to the partly neutralized product, ofthe acrylic acid to prepare an unsaturated aqueous solution of monomerfor the first-stage polymerization.

The entire unsaturated aqueous solution of monomer for the first-stagepolymerization was added and dispersed in the five-necked cylindricalround bottom flask mentioned above, while stirring the content of thefive-necked cylindrical round bottom flask. Nitrogen gas was introducedinto the system until it sufficiently substitutes the inner gas. Theflask was then heated to 50° C. and maintained in a bath at 70° C. tolet a polymerization reaction proceed for 1 hour. The flask was cooleddown to room temperature to obtain a liquid of polymerization slurry.

Another triangle flask of a capacity of 500 mL was charged with 119.1 gof an 80 mass % aqueous solution of acrylic acid (the acrylic acid wasthe aforementioned commercially available acrylic acid (special gradereagent containing 200 ppm p-methoxyphenol, manufactured by Wako PureChemical Ind.)). 132.2 g of a 30 mass % aqueous solution of sodiumhydroxide (iron content: 0.2 ppm) was added dropwise to the triangleflask while the flask was being cooled down to neutralize 75 mol % ofthe acrylic acid. Then, 27.4 g of water, 0.14 g of potassium persulfate,and 35.7 mg of ethylene glycol diglycidyl ether were added to prepare anunsaturated aqueous solution of monomer for the second-stagepolymerization. The flask was cooled in an iced water bath.

The unsaturated aqueous solution of monomer for the second-stagepolymerization was entirely added to the liquid of polymerizationslurry. Thereafter, nitrogen gas was introduced into the system until itsufficiently substitutes the inner gas. The bath was then maintained at70° C. to let a second-stage polymerization reaction proceed for 2hours. After the termination of the polymerization reaction, water wasremoved from the system (water-containing gel-like substance dispersedin n-heptane) by azeotropic distillation with the n-heptane. 8.44 g of a2 mass. % aqueous solution of ethylene glycol diglycidyl ether was addedto the obtained gel-like substance. Then, water and the n-heptane wereremoved by distillation to dry the substance. 215.5 g of awater-absorbent resin was thus obtained. Comparative composition (D-18)was obtained as in comparative example 1.

Example 10

Water-absorbent resin composition (D-4′) was prepared by following thesame procedures as in example 4, except that trisodiumdiethylenetriamine pentacetate in mixed solution (b-4) was used insteadof salt of trisodium methyl glycine diacetate. Measurements of thecoloring values, degraded soluble component, and degraded liquidpermeation rate of water-absorbent resin composition (D-4′) are shown inTable 1.

Example 11

Water-absorbent resin composition (D-19) was prepared by following thesame procedures as in example 1, except that 1.788 mass parts ofpreparation solution (a) was added to and mixed with 100 mass parts ofthe water-absorbent resin obtained in reference example 3 instead ofadding and mixing 1.192 mass parts of preparation solution (a) with 100mass parts of the water-absorbent resin obtained in reference example 1.

The obtained water-absorbent resin composition showed a centrifugeretention capacity (CRC) of 27 g/g, an absorbency against pressure (AAP)of 24 g/g, and a saline flow conductivity (SFC) of 100×10⁻⁷·cm³·s·g⁻¹.Besides, the composition showed a mass-average particle diameter (D50)of 420 μm and a logarithmic standard deviation (σζ) of 0.42. The resincontained particles sized 150 to 850 μm in a proportion of at least96.1% to the resin composition. Measurements of the coloring values,degraded soluble component, and degraded liquid permeation rate ofwater-absorbent resin composition (D-19) are shown in Table 1.

TABLE 1 Multivalent Water- Metal Chelating Degraded Degraded absorbingCations Fe²⁺ Agent Soluble Liquid Resin (wt %) (ppm) (ppm) ColoringValues Component Permeability Composition *1 *2 *1 L a b (%) Rate (sec.)Example 1 D-1 0.044 227 — 90.1 −0.52 6.0 27.9 33 Example 2 D-2 0.0442273 — 90.2 −0.49 6.0 28.2 32 Example 3 D-3 0.044 22727 — 90.1 −0.45 6.128.2 33 Example 4 D-4 0.044 2273 3 90.3 −0.39 5.5 22.9 31 Example 5 D-50.044 22727 30 90.2 −0.49 5.8 23.0 31 Example 6 D-6 0.044 113636 15090.2 −0.43 6.3 22.6 33 Example 7 D-7 0.044 227273 300 90.4 −0.54 6.522.9 33 Example 8 D-8 0.044 454545 600 90.3 −0.60 7.1 27.4 30 Comp. Ex.1 D-9 0.044 113636 — 90.1 −0.43 6.4 29.2 42 Comp. Ex. 2 D-10 0.044227273 — 90.4 −0.50 6.8 32.3 50 Comp. Ex. 3 D-11 0.044 454545 — 88.9−0.55 8.3 35.0 52 Comp. Ex. 4 D-12 0.044 1136364 — 85.1 −0.51 12.3 39.483 Comp. Ex. 5 D-13 0.044 2272727 — 81.3 −0.57 14.5 48.4 178 Comp. Ex. 6D-14 0.044 1136364 1500 88.1 −0.70 11.2 36.9 52 Comp. Ex. 7 D-15 0.0442272727 3000 84.8 −0.67 13.1 43.1 168 Example 9 D-16 0.044 227 — 90.5−0.49 5.9 30.1 45 Comp. Ex. 8 D-17 0.044 227 — 85.5 −0.63 13.2 44.2 171Comp. Ex. 9 D-18 0.044 113636 — 85.3 −0.65 13.4 29.2 42 Example 10 D-4′0.044 2273 3 90.2 −0.41 5.6 23.0 30 Example 11 D-19 0.066 340.5 — 90.0−0.51 6.1 19.9 25 *1 Concentration to water-absorbent resin inwater-absorbent resin composition *2 Concentration to aluminum cationsin water-absorbent resin composition

As shown in Table 1, the water-absorbent resin compositions of examples1 to 3 differ from those of comparative examples 1 to 5 in that thewater-absorbent resin compositions of examples 1 to 3 contain Fe cationsin an amount of 50,000 ppm or less to the multivalent metal, cationsother than the Fe cations, thereby containing less of the degradedsoluble component and showing a higher degraded liquid permeation rate.The former also exhibit a greater coloring value (L value), indicatingthat the compositions will color less. These effects are especiallyevident in examples 1 and 2 because the Fe cations account for 0.1 ppmof the water-absorbent resin in example 1 and 1 ppm in example 2.

The water-absorbent resin compositions of examples 4 to 9, containing achelating agent, differ from those of comparative example 6, 7. Theformer contain Fe cations in an amount of 500,000 ppm or less to themultivalent metal cations other than the Fe cations, thereby containingless of the degraded soluble component and showing a higher degradedliquid permeation rate. The former also exhibit a greater coloring value(L value), indicating that the compositions will color less.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

The embodiments and concrete examples of implementation, discussed inthe foregoing detailed explanation serve solely to illustrate thetechnical details of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied, in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

INDUSTRIAL APPLICABILITY

The water-absorbent resin composition in accordance with the presentinvention is, as described in the foregoing, shows only small reductionin liquid permeability and limited coloring over time or in relation toanother factor. The composition is therefore suitable for use as anabsorbent substance in disposable diapers, as an example.

1. A water-absorbent resin composition being characterized in that thecomposition comprises: a polycarboxylate-based water-absorbent resin asa primary component, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the multivalent metal cations otherthan Fe cations account for 0.001 to 1 mass % of the water-absorbentresin; and the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is less than or equal to 5.00 mass %.
 2. Awater-absorbent resin composition being characterized in that thecomposition comprises: a polycarboxylate-based water-absorbent resin asa primary component, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the multivalent metal cations otherthan Fe cations account for 0.001 to 1 mass % of the water-absorbentresin; and the ratio of the Fe cations to the water-absorbent resin isnot more than 1 ppm.
 3. The water-absorbent resin composition of claim1, wherein the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is 1 to 3,000 ppm.
 4. A water-absorbent resincomposition being characterized in that the composition comprises: apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; multivalent metal cations; and achelating agent, wherein: the multivalent metal cations other than Fecations account for 0.001 to 1 mass % of the water-absorbent resin; andthe ratio of the Fe cations to the multivalent metal cations other thanthe Fe cations is less than or equal to 50 mass %.
 5. A water-absorbentresin composition being characterized in that the composition comprises:a polycarboxylate-based water-absorbent resin as a primary component,the resin having a crosslinked structure formed by polymerization of anacid group-containing unsaturated monomer; multivalent metal cations;and a chelating agent, wherein: the multivalent metal cations other thanFe cations account for 0.001 to 1 mass % of the water-absorbent resin;and the ratio of the Fe cations to the water-absorbent resin is not morethan 1 ppm.
 6. The water-absorbent resin composition of claim 5, whereinthe ratio of the Fe cations to the multivalent metal cations other thanthe Fe cations is 200 to 5,000 ppm.
 7. The water-absorbent resincomposition of claim 1, wherein: the composition is of a particulateshape and contains the multivalent metal cations on surfaces thereof;and the composition is surface crosslinked by a surface crosslinkingagent other than the metal cations.
 8. The water-absorbent resincomposition of claim 1, wherein: the multivalent metal cations arealuminum cations.
 9. The water-absorbent resin composition of claim 1,wherein the composition has a degraded liquid permeation rate of greaterthan 0 and less than or equal to 40 seconds.
 10. The water-absorbentresin composition of claim 1, wherein the composition contains a 0 to 30mass % degraded soluble component.
 11. The water-absorbent resincomposition of claim 1, wherein the composition shows a coloring value(L value) of more than or equal to 90.0.
 12. The water-absorbent resincomposition of claim 1, wherein: the composition is of a particulateshape; and the composition has a mass-average particle diameter of 250to 600 μm and contains 90 to 100 mass % particles that have a particlediameter of 150 to 850 μm.
 13. The water-absorbent resin composition ofclaim 1, wherein the acid group-containing unsaturated monomer contains10 to 180 ppm methoxyphenol.
 14. A method of manufacturing awater-absorbent resin composition being characterized in that the methodcomprises the steps of: (a) polymerizing an acid group-containingunsaturated monomer into a polycarboxylate-based water-absorbent resinwith a crosslinked structure, the monomer including an acrylic acidand/or a salt thereof as primary components; and (b) adding multivalentmetal cations to the water-absorbent resin in 0.001 to 5 mass % to thewater-absorbent resin, the ratio of Fe cations to the multivalent metalcations other than the Fe cations being less than or equal to 0.50 mass%.
 15. The method of claim 12, further comprising the step of (c)surface crosslinking the water-absorbent resin by a surface crosslinkingagent other than the multivalent metal cations, step (c) being differentfrom step (b), wherein step (b) is carried out either in or after step(c).
 16. The method of claim 14, further comprising the step of (d)adding a chelating agent to the water-absorbent resin, wherein step (d)is carried out either in or after step (a).
 17. The method of claim 14,further comprising the step of (e) adjusting a methoxyphenol content inthe acid group-containing unsaturated monomer used in step (a) to 10 to180 ppm.
 18. An absorbent article being characterized in that thearticle is at least one absorbent article selected from the groupconsisting of a paper diaper, a sanitary napkin, and an incontinencepad, the article comprising a water-absorbent resin compositioncontaining: a polycarboxylate-based water-absorbent resin as a primarycomponent, the resin having a crosslinked structure formed bypolymerization of an acid group-containing unsaturated monomer; andmultivalent metal cations, wherein: the multivalent metal cations otherthan Fe cations account for 0.001 to 1 mass % of the water-absorbentresin; and the ratio of the Fe cations to the multivalent metal cationsother than the Fe cations is less than or equal to 5.00 mass %.
 19. Anabsorbent article being characterized in that the article is at leastone absorbent article selected from the group consisting of a paperdiaper, a sanitary napkin, and an incontinence pad, the articlecomprising a water-absorbent resin composition containing: apolycarboxylate-based water-absorbent resin as a primary component, theresin having a crosslinked structure formed by polymerization of an acidgroup-containing unsaturated monomer; multivalent metal cations; and achelating agent, wherein: the multivalent metal cations other than Fecations account for 0.001 to 1 mass % of the water-absorbent resin; andthe ratio of the Fe cations to the multivalent metal cations other thanthe Fe cations is less than or equal to 50 mass %.
 20. Thewater-absorbent resin composition of claim 2, wherein: the compositionis of a particulate shape and contains the multivalent metal cations onsurfaces thereof; and the composition is surface crosslinked by asurface crosslinking agent other than the metal cations.
 21. Thewater-absorbent resin composition of claim 4, wherein: the compositionis of a particulate shape and contains the multivalent metal cations onsurfaces thereof; and the composition is surface crosslinked by asurface crosslinking agent other than the metal cations.
 22. Thewater-absorbent resin composition of claim 5, wherein: the compositionis of a particulate shape and contains the multivalent metal cations onsurfaces thereof; and the composition is surface crosslinked by asurface crosslinking agent other than the metal cations.
 23. Thewater-absorbent resin composition of claim 2, wherein: the multivalentmetal cations are aluminum cations.
 24. The water-absorbent resincomposition of claim 4, wherein: the multivalent metal cations arealuminum cations.
 25. The water-absorbent resin composition of claim 5,wherein: the multivalent metal cations are aluminum cations.
 26. Thewater-absorbent resin composition of claim 2, wherein the compositionhas a degraded liquid permeation rate of greater than 0 and less than orequal to 40 seconds.
 27. The water-absorbent resin composition of claim4, wherein the composition has a degraded liquid permeation rate ofgreater than 0 and less than or equal to 40 seconds.
 28. Thewater-absorbent resin composition of claim 5, wherein the compositionhas a degraded liquid permeation rate of greater than 0 and less than orequal to 40 seconds.
 29. The water-absorbent resin composition of claim2, wherein the composition contains a 0 to 30 mass % degraded solublecomponent.
 30. The water-absorbent resin composition of claim 4, whereinthe composition contains a 0 to 30 mass % degraded soluble component.31. The water-absorbent resin composition of claim 5, wherein thecomposition contains a 0 to 30 mass % degraded soluble component. 32.The water-absorbent resin composition of claim 2, wherein thecomposition shows a coloring value (L value) of more than or equal to90.0.
 33. The water-absorbent resin composition of claim 4, wherein thecomposition shows a coloring value (L value) of more than or equal to90.0.
 34. The water-absorbent resin composition of claim 5, wherein thecomposition shows a coloring value (L value) of more than or equal to90.0.
 35. The water-absorbent resin composition of claim 2, wherein: thecomposition is of a particulate shape; and the composition has amass-average particle diameter of 250 to 600 μm and contains 90 to 100mass % particles that have a particle diameter of 150 to 850 μm.
 36. Thewater-absorbent resin composition of claim 4, wherein: the compositionis of a particulate shape; and the composition has a mass-averageparticle diameter of 250 to 600 μm and contains 90 to 100 mass %particles that have a particle diameter of 150 to 850 μm.
 37. Thewater-absorbent resin composition of claim 5, wherein: the compositionis of a particulate shape; and the composition has a mass-averageparticle diameter of 250 to 600 μm and contains 90 to 100 mass %particles that have a particle diameter of 150 to 850 μm.
 38. Thewater-absorbent resin composition of claim 2, wherein the acidgroup-containing unsaturated monomer contains 10 to 180 ppmmethoxyphenol.
 39. The water-absorbent resin composition of claim 4,wherein the acid group-containing unsaturated monomer contains 10 to 180ppm methoxyphenol.
 40. The water-absorbent resin composition of claim 5,wherein the acid group-containing unsaturated monomer contains 10 to 180ppm methoxyphenol.